Methods comparison for detecting trends in herbicide monitoring time-series in streams.

An inadvertent consequence of pesticide use is aquatic pesticide pollution, which has prompted the implementation of mitigation measures in many countries. Water quality monitoring programs are an important tool to evaluate the efficacy of these mitigation measures. However, large interannual variability of pesticide losses makes it challenging to detect significant improvements in water quality and to attribute these improvements to the application of specific mitigation measures. Thus, there is a gap in the literature that informs researchers and authorities regarding the number of years of aquatic pesticide monitoring or the effect size (e.g., loss reduction) that is required to detect significant trends in water quality. Our research addresses this issue by combining two exceptional empirical data sets with modelling to explore the relationships between the achieved pesticide reduction levels due to mitigation measures and the length of the observation period for establishing statistically significant trends. Our study includes both a large (Rhine at Basel, ∼36,300 km2) and small catchment (Eschibach, 1.2 km2), which represent spatial scales at either end of the spectrum that would be realistic for monitoring programs designed to assess water quality. Our results highlight several requirements in a monitoring program to allow for trend detection. Firstly, sufficient baseline monitoring is required before implementing mitigation measures. Secondly, the availability of pesticide use data helps account for the interannual variability and temporal trends, but such data are usually lacking. Finally, the timing and magnitude of hydrological events relative to pesticide application can obscure the observable effects of mitigation measures (especially in small catchments). Our results indicate that a strong reduction (i.e., 70-90 %) is needed to detect a change within 10 years of monitoring data. The trade-off in applying a more sensitive method for change detection is that it may be more prone to false-positives. Our results suggest that it is important to consider the trade-off between the sensitivity of trend detection and the risk of false positives when selecting an appropriate method and that applying more than one method can provide more confidence in trend detection.

Mode of action-based classification and prediction of activity of uncouplers for the screening of chemical inventories.

A new approach for classification of uncouplers of oxidative and photophosphorylation, also suitable for screening of large chemical inventories, is introduced. Earlier fragment-based approaches for this mode of toxic action are limited to phenols but weak acids of extremely diverse chemical classes can act as uncouplers. The proposed approach overcomes the limitation to phenolic uncouplers by combining structural fragments with the global information of physico-chemical descriptors. In a top-down approach to reduce the number of candidate chemicals, firstly substructure definitions for the detection of weak acids were applied. Subsequently, conservative physico-chemical thresholds for the two most important properties for the uncoupling activity were defined: an acid dissociation constant (pK(a)) between 3 and 9, and a sufficiently low energy barrier for the internal permeability of anions (17 kcal/mol). The later was derived from a novel approach to calculate the distribution of compounds across membranes. The combination of structural and physico-chemical criteria allowed a good separation of active from inactive chemicals with high sensitivity (95%) and slightly lower (more than 75%) specificity. Applying this approach to several thousand high and low production volume chemicals retrieved a surprisingly small number of 10 compounds with a predicted excess toxicity above 10. Nevertheless, uncoupling can be an important mode of action as highlighted with several examples ranging from pesticide metabolites to persistent organic compounds.

A novel human tocopherol-associated protein: cloning, in vitro expression, and characterization.

Vitamin E (alpha-tocopherol) is an essential dietary nutrient for humans and animals. The mechanisms involved in cellular regulation as well as in the preferential cellular and tissue accumulation of alpha-tocopherol are not yet well established. We previously reported (Stocker, A., Zimmer, S., Spycher, S. E., and Azzi, A. (1999) IUBMB Life 48, 49-55) the identification of a novel 46-kDa tocopherol-associated protein (TAP) in the cytosol of bovine liver. Here, we describe the identification, the molecular cloning into Escherichia coli, and the in vitro expression of the human homologue of bovine TAP, hTAP. This protein appears to belong to a family of hydrophobic ligand binding proteins, which have the CRAL (cis-retinal binding motif) sequence in common. By using a biotinylated alpha-tocopherol derivative and the IASys resonant mirror biosensor, the purified recombinant protein was shown to bind tocopherol at a specific binding site with K(d) 4.6 x 10(-7) m. Northern analyses showed that hTAP mRNA has a size of approximately 2800 base pairs and is ubiquitously expressed. The highest amounts of hTAP message are found in liver, brain, and prostate. In conclusion, hTAP has sequence homology to proteins containing the CRAL_TRIO structural motif. TAP binds to alpha-tocopherol and biotinylated tocopherol, suggesting the existence of a hydrophobic pocket, possibly analogous to that of SEC14.

Specific cellular responses to alpha-tocopherol.

In the last 10 years precise cellular functions of alpha-tocopherol, some of which are independent of its antioxidant/radical-scavenging ability, have been revealed. Absorption of alpha-tocopherol from the gut is a selective process. Other tocopherols are not absorbed or are absorbed to a lesser extent. At the post-translational level, alpha-tocopherol inhibits protein kinase C and 5-lipoxygenase and activates protein phosphatase 2A and diacylglycerol kinase. Some genes [platelet glycoprotein IV/thrombospondin receptor/class B scavenger receptor (CD36), alpha-tocopherol transfer protein (alpha-TTP), alpha-tropomyosin, connective tissue growth factor and collagenase] are affected by alpha-tocopherol at the transcriptional level. alpha-Tocopherol also inhibits cell proliferation, platelet aggregation, monocyte adhesion and the oxygen burst in neutrophils. Other antioxidants, such as beta-tocopherol and probucol, do not mimic these effects, suggesting a nonantioxidant, alpha-tocopherol-specific molecular mechanism.

Modulation of alpha-tropomyosin expression by alpha-tocopherol in rat vascular smooth muscle cells.

The effect of alpha-tocopherol (vitamin E) on gene expression in rat vascular smooth muscle cells was studied by the differential display technique. One gene out of about 1000 genes analyzed, identified as alpha-tropomyosin, showed an increased transcription level caused by alpha-tocopherol treatment. Northern and Western blot analysis revealed a time-dependent transient up-regulation of the amount of mRNA (peak between 2 and 3 h) and protein (peak at 5 h) in alpha-tocopherol-treated cells. No effect was observed in cells treated with beta-tocopherol.

Signal transduction in rat vascular smooth muscle cells: control of osmotically induced aldose reductase expression by cell kinases and phosphatases.

We have studied the osmotically induced gene expression (measured as chloramphenicol acetyl transferase (CAT) reporter gene expression) in rat smooth muscle cell primary cultures (rSMC), under the control of osmotic response elements (ORE). It was found that osmotically induced gene expression is sensitive to signal transduction inhibitors and activators. In particular, protein kinase C inhibition by calphostin C prevented gene expression by osmotic response. On the other hand, receptor tyrosine kinase inhibition has been shown to produce an enhancement of gene expression. This suggests that tyrosine kinase receptor activation exerts an inhibitory action on ORE induced gene expression. Gene expression was also induced by treating cells with PD098059, a specific inhibitor of mitogen-activated protein kinase kinase. Moreover, the same inhibitors and activators have been shown to affect the hyperosmosis induced expression of aldose reductase gene.

Identification of a novel cytosolic tocopherol-binding protein: structure, specificity, and tissue distribution.

Alpha-tocopherol plays an important role as a lipid-soluble antioxidant. It is present in all major mammalian cell types and shows tissue-specific distribution. This suggests the presence of specific proteins involved in intracellular distribution or metabolism of alpha-tocopherol. A diminution of tocopherol plasma concentrations contributes to the development of diseases such as vitamin E deficiency (AVED), atherosclerosis, and prostate cancer. Further evidence has been obtained for the existence of sites in cellular metabolism and signal transduction where alpha-tocopherol potentially plays a regulatory role. A signal transduction modulation specific for alpha-tocopherol has been described in several model systems. Using radioactively labeled alpha-tocopherol as tracer, we have isolated a new alpha-tocopherol-associated protein (TAP) from bovine liver. This protein has a molecular mass of 46 kDa and an isoelectric point of 8.1. From its partial amino acid sequence, a human gene has been identified with high homology to the newly described protein. Sequence analysis has established that the new TAP has structural motifs suggesting its belonging to a family of hydrophobic ligand-binding proteins (RALBP, CRALBP, alpha-TTP, SEC 14, PTN 9, RSEC 45). Human TAP has been cloned into Escherichia coli, and its tissue-specific expression has been assessed by Northern blot analysis.

RRR-alpha-tocopherol regulation of gene transcription in response to the cell oxidant status.

RRR-alpha-Tocopherol, but not RRR-beta-tocopherol, negative regulates proliferation of vascular smooth muscle cells at physiological concentrations. At the same concentrations RRR-alpha-tocopherol inhibits protein kinase C activity, whereas RRR-beta-tocopherol is ineffective. Furthermore, RRR-beta-tocopherol prevents the inhibition of cell growth and of protein kinase C activity caused by RRR-alpha-tocopherol. The negative regulation by RRR-alpha-tocopherol of protein kinase C activity appears to be the cause of smooth muscle cell growth inhibition. RRR-alpha-Tocopherol does not act by binding to protein kinase C directly but presumably by preventing protein kinase C activation. A second RRR-alpha-tocopherol effect has been found at the level of AP 1, the latter becoming activated by RRR-alpha-tocopherol under condition of protein kinase C inhibition or down regulation. AP-1 inhibition by RRR-alpha-tocopherol is seen, however, under condition of protein kinase C stimulation. Compositional changes of AP-1 have been found to be at the basis of the RRR-alpha-tocopherol effects. RRR-beta-tocopherol, provided with similar antioxidant properties, not only it does not affect AP 1 but it prevents the effects of RRR-alpha-tocopherol. Moreover, it has been observed that RRR-alpha-tocopherol is able to affect TRE regulated gene transcription. It is concluded that RRR-alpha-tocopherol acts specifically in vascular smooth muscle cells, by controlling a signal transduction pathway leading to cell proliferation by a non-antioxidant mechanism.

Molecular basis of alpha-tocopherol control of smooth muscle cell proliferation.

Rat and human vascular smooth muscle cell proliferation is specifically sensitive to alpha-tocopherol, but not beta-tocopherol. The former, but not the latter, is capable of limiting proliferation and inhibiting protein kinase C activity in a dose-dependent manner. The phenomenon occurs at concentrations in the range 10-50 microM. beta-tocopherol addition together with alpha-tocopherol, prevents both cell growth and protein kinase C inhibition. alpha-tocopherol increases de novo synthesis of protein kinase C molecules. The enzyme specific activity, however, is diminished, due to a decreased phosphorylation of protein kinase C, occurring in the presence of alpha-tocopherol. Experiments with protein kinase C isoform-specific inhibitors and precipitating antibodies show that the only isoform affected by alpha-tocopherol is protein kinase C-alpha. The effect of alpha-tocopherol is prevented by okadaic acid indicating a phosphatase of the PP2A type as responsible for protein kinase C-alpha dephosphorylation produced in the presence of alpha-tocopherol. At a gene level alpha-tocopherol but not beta-tocopherol induces a transient activation of alpha-tropomyosin gene transcription and protein expression. It is proposed that, by inhibiting protein kinase C activity via an activation of a phosphatase PP2A, alpha-tocopherol controls smooth muscle cell proliferation through changes in gene expression.

Aldose reductase induction: a novel response to oxidative stress of smooth muscle cells.

Hydrogen peroxide (H2O2) or 4-hydroxy-2,3-trans-nonenal (HNE) treatment of rat vascular smooth muscle cells (A7r5) caused induction of aldose reductase mRNA. Induction was dose (10-100 microM H2O2, 1-10 microM HNE) and time dependent, reaching a maximum (three- to fourfold) after 7-12 h. Treatment of cells with actinomycin D confirmed de novo synthesis of aldose reductase mRNA. H2O2-induced expression was prevented by catalase but unaffected by Desferal, indicating that metal catalyzed degradation of peroxide was not involved. Induction of enzymatically active aldose reductase by H2O2 and HNE was confirmed using Western blotting and enzyme assays. Aldose reductase can metabolize several aldehyde compounds including HNE, a major toxic product of lipid peroxidation. Inclusion of Sorbinil, an aldose reductase inhibitor, in toxicity assays resulted in a significant (twofold) enhancement of HNE-mediated killing of A7r5 cells, suggesting a protective role of aldose reductase against HNE-induced cell death. These data indicate that the induction of aldose reductase during oxidative stress might represent an important cellular antioxidant defense mechanism.

4-hydroxy-2,3-trans-nonenal induces transcription and expression of aldose reductase.

Treatment of A7r5 rat vascular smooth muscle cells with 4-hydroxy-2,3-trans-nonenal (HNE) resulted in increased aldose reductase mRNA transcription. Induction was time dependent, reaching a maximum (2,3-fold) after 7 hours. An enzymatically active aldose reductase analysed by Western blotting and enzyme assays was expressed. Sorbinil, an aldose reductase inhibitor, induced a significant enhancement of HNE cytotoxicity, indicating a protective role of aldose reductase against HNE-induced A7r5 cell death. These data indicate that induction of aldose reductase by HNE may represent an important cellular defence mechanism against oxidative injury.

The role of hydrogen peroxide and RRR-alpha-tocopherol in smooth muscle cell proliferation.

Oxidants can be considered early growth signals, since they have been shown to activate a number of pathways that are also stimulated by growth factors. In particular, H(2)O(2) activates the protein kinase C signal transduction pathway in smooth muscle cells. These events certainly play a role in the activation of the DNA synthesis machinery although it is still unclear whether they can also regulate the lethal response. Evidence exists of an oxidant-mediated increase in tyrosine protein phosphorylation as an early event in the signal transduction cascade of growth factor receptors, leading to augmentation of cell proliferation. Oxidants can also induce transcription of enzymes, such as ornithine decarboxylase and the phosphatase CL-100. CL-100 is the first example of a new class of protein phosphatases responsible for modulating the activation of MAP kinase following exposure of quiescent cells to growth factors and further implicates MAP kinase activation/deactivation in the cellular response to hydrogen peroxide. Moreover H(2)O(2) activates the MAP kinase cascade by stimulating the tyrosine kinase and protein kinase C pathways. JNK1, a relative of the MAP kinase group, is activated by dual phosphorylation at Thr and Tyr during the UV response. RRR-alpha-tocopherol and RRR-beta-tocopherol have different and competing effects on smooth muscle cell proliferation, indicating that they do not act as antioxidants. The earliest event brought by RRR-alpha-tocopherol in the signal transduction cascade contolling receptor mediated cell growth is the inhibition of the transcription factor AP-1, activated by phorbol esters. RRR-beta-tocopherol alone is without effect but in combination with RRR-alpha-tocopherol prevents the AP-1-inhibiting effect of the latter. Protein kinase C is inhibited by RRR-alpha-tocopherol and not by RRR-beta-tocopherol, which also in this case prevented the effect of RRR-alpha-tocopherol. The inhibition of RRR-alpha-tocopherol of protein kinase C is not the consequence of a direct interaction but is due to a diminution, produced by RRR-alpha-tocopherol of the kinase phosphorylation. A tocopherol binding protein appears to be at the basis of the RRR-alpha-tocopherol, that discriminates between RRR-alpha-tocopherol and RRR-beta-tocopherol and initiates a cascade of events at the level of cell signal transduction leading to cell proliferation inhibition.

Involvement of oxidants and oxidant-generating enzyme(s) in tumour-necrosis-factor-alpha-mediated apoptosis: role for lipoxygenase pathway but not mitochondrial respiratory chain.

Cellular signalling by the inflammatory cytokine tumour necrosis factor alpha (TNF alpha) has been suggested to involve generation of low levels of reactive oxygen species (ROS). Certain antioxidants and metal chelators can inhibit cytotoxicity and gene expression in response to TNF alpha in numerous cell types. However, neither the source nor function of TNF alpha-induced oxidant generation is known. Using specific inhibitors, we ruled out involvement of several oxidant-generating enzymes [cyclo-oxygenase (indomethacin), cytochrome P-450 (metyrapone), nitric oxide synthase (NG-methyl-L-arginine), NADPH oxidase (iodonium diphenyl), xanthine oxidase (allopurinol), ribonucleotide reductase (hydroxyurea)] in TNF alpha-mediated apoptosis of the murine fibrosarcoma line, L929. We also demonstrated no role for mitochondrial-derived radicals/respiratory chain in the lytic pathway using specific inhibitors/uncouplers (rotenone, KCN, carboxin, fluoroacetate, antimycin, malonate, carbonyl cyanide p-trifluoromethoxyphenylhydrazone) and chloramphenicol-derived respiration-deficient cells. Significant ROS (H2O2, O2-.) generation was not observed in response to TNF alpha in L929 cells using four separate assays. Also, prevention of intracellular H2O2 removal by inhibition of catalase did not potentiate TNF alpha-mediated cell death. These data suggest that neither H2O2 nor O2-. plays a direct role in TNF alpha cytotoxicity. Finally, we suggest a central role for lipoxygenase in TNF alpha-mediated lysis. Three inhibitors of this radical-generating signalling pathway, including an arachidonate analogue (5,8,11,14-eicosatetraynoic acid), could protect cells against TNF alpha. The inhibitor nordihydroguaiaretic acid is also a radical scavenger, but it could not protect cells from ROS toxicity at concentrations that effectively prevented TNF alpha killing. Therefore protection by nordihydroguaiaretic acid cannot be due to scavenging of cytotoxic H2O or O2-.. The lipoxygenase product, (12S)-hydroxyeicosatetraenoic acid, was also significantly protective. As this analogue can act as a substrate for certain lipoxygenases, this effect may be due to prevention of generation of physiological products.

Mapping of amino acid residues in the C mu 3 domain of mouse IgM important in macromolecular assembly and complement-dependent cytolysis.

By analyzing the effects of single site mutations of a TNP-binding mouse IgM we have identified amino acid residues clustered in two regions in the C mu 3 domain that are important in the complement-dependent cytolytic activity of polymeric IgM. Some of the mutations also impaired IgM polymerization. For one of these clusters, D432G, P434A, and P436S, which lies on the fy2 and fy3 strands and their connecting loop, polymerization was little affected and the effect on the cytolytic activity of the polymer fraction was taken to imply direct involvement of the residue in C1 binding. The other cluster, involving residues D356A K361A and D417G, is situated at the other end of the C mu 3 domain closer to the center of the Fc mu disc. The D356A K361A and D417G mutations significantly impaired polymer formation, suggesting that these residues are necessary for proper folding or packing of the C mu 3 domains and may affect cytolysis only indirectly. Some other mutations had little or no effect on polymerization or cytolytic activity (E423A, E527G), whereas some mutations impaired only IgM polymerization without affecting cytolytic activity (D344A, K361A, K443A P544G). In others the defect in polymerization was so profound that only the monomer formed (H430A/N/Q and K438G). Our results also suggest that the C1 binding site of IgM is not strictly homologous to the C1 binding site of IgG. Although mutation of E318 of IgG has been shown to reduce its cytolytic activity, mutation of the homologous residue in IgM, E423, was without effect as were mutations of other flanking-charged residues. Proline at 436 in IgM and 331 in IgG may, however, be a common element.

The mitochondrial tricarboxylate carrier.

The tricarboxylate carrier has recently been purified from rat liver mitochondria by three distinct scientific groups using different methods. A 37-38-kDa protein has been prepared by silca gel 60 chromatography by our group (Claeys and Azzi, 1989; Glerum et al., 1990). The specific citrate transport activity of this preparation is not significantly different from that measured in mitochondria and it is inhibitable by 1,2,3-benzenetricarboxylic acid. Bisaccia et al. (1990) have reported the isolation of a 30-kDa protein by Celite 535 chromatography, and Kaplan's group (Kaplan et al., 1990) have isolated a 32.5-kDa protein by Matrex Orange, Matrex Blue, and Affi-Gel chromatography. Peptide mapping has failed to support any structural homologies between the 37-38-kDa and the 30-32.5-kD proteins. The 38-kD protein is N-terminally blocked. The peptides obtained by several cleavage procedures have been partially sequenced. Their sequence information has been used to obtain different cDNA clones by a dual approach, the polymerase chain reaction and screening of a lambda ZAP cDNA library. The largest cDNA which could be isolated is 2,986 bp in length and contains a 1071-bp-long open reading frame and an unusually long 3' untranslated region, both of which have been completely sequenced. The protein sequence of the carrier from the first in-frame methionine is 322 amino acids in length and exhibits a molecular mass of 35,546. Comparison of the protein sequence to the sequences of the four members of the mitochondrial carrier protein family (ADP/ATP carrier, phosphate carrier, 2-oxoglutarate/malate carrier, and uncoupling protein) does not reveal significant similarity (cf. Walker et al., 1987). A tripartite internal homology, which is a characteristic of these proteins, is not present in the sequence of the tricarboxylate carrier protein. The mRNA for the tricarboxylate carrier is expressed in rat liver and brain, but not in rat heart.

The validity of the MIMIC (Multiple Indicators/MultIple Causes) health index--some empirical evidence.

This study evaluates the potential of econometric models with latent (unobservable) variables for measuring health or health impairment due to a specific disease. A MIMIC disability index is estimated for a sample of 145 adults with chronic bronchitis, expressing their self-reported disability caused by the disease on a one-dimensional scale. The index is determined up to a linear transformation. Disability is thus measured on an interval scale. The data were collected by interviews. The questionnaire used for this purpose is based on a number of in-depth interviews with selected bronchitis patients conducted beforehand. The study therefore focuses directly on the patients' perceptions of their disease. The validity of the index is evaluated in three different ways. First, construct validity is assessed performing groupwise analysis and testing for differences in the index values by subgroup. To a large extent, the index is consistent with a priori expectations. Therefore, we conclude that it has high construct validity. Second, validity of the index is assessed by comparing its results to a direct rating scale produced by 21 physicians with various medical backgrounds. The MIMIC index turns out to be related in a systematic, but nonlinear way to this direct rating scale. This can be interpreted in two different ways. If one accepts the preferences of health providers as the ultimate yardstick when it comes to ranking health or chronic states the result suggests that the MIMIC index estimated in this way is not a valid measure of treatment success. By contrast, if patients' preferences are considered to be decisive, it suggests that physician-based ratings should be substituted for or at least complemented with patient-based indices (such as the MIMIC disability index estimated here) when evaluating medical services in terms of cost-effectiveness. Third we explore the extent to which the MIMIC index reflects utility associated with different states of disability, using a modified Torrance Standard Gamble approach. The above-mentioned physicians are used as experts in this procedure. The results indicate that the MIMIC index as estimated here is related in a systematic, but nonlinear way to the Standard Gamble risk index as well. The fact that this relationship is nonlinear indicates that the MIMIC index does not measure utility as derived from the experts' preferences directly. How this index would fare compared to a Standard Gamble risk index provided by patients (bronchitis subjects) is a question which remains open.(ABSTRACT TRUNCATED AT 400 WORDS)

A functional, thioester-containing alpha 2-macroglobulin homologue isolated from the hemolymph of the American lobster (Homarus americanus).

An alpha 2-macroglobulin-like protease inhibitor was isolated from the cell-free hemolymph of the american lobster (Homarus americanus) by ion-exchange chromatography and gel filtration. Whereas the undissociated molecule has a molecular weight of 342,000 as determined by ultracentrifugation studies, under reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the protein has a subunit molecular weight of 180,000. On the basis of this and other evidence, we conclude that the lobster protein is a dimer consisting of two disulfide-bonded monomers. The purified protein inhibits proteolytic enzymes but protects the esterolytic activity of trypsin toward low molecular weight substrates from inactivation by soybean trypsin inhibitor. The methylamine sensitivity of this activity suggests the presence of an internal thioester bond. This was confirmed by the covalent incorporation of [14C]methylamine, by the formation of Mr 55,000 and 125,000 autolytic cleavage fragments in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and, more directly, by the amino acid sequence of a tryptic peptide containing the putative thioester region. Whereas the N-terminal amino acid sequence (22 residues) of the protein revealed an overall identity of only 18% when compared with the human protein, the sequence of the thioester-containing peptide was highly conserved, both with respect to human alpha 2-macroglobulin and to other proteins having a thioester bond. The protein showed the "slow to fast" conformational change typical in alpha 2-macroglobulins in nondenaturing gel electrophoresis after treatment with trypsin, but not after incubation with methylamine.

Human complement component C1s. Partial sequence determination of the heavy chain and identification of the peptide bond cleaved during activation.

Human C1s proenzyme (Mr 83 000) was isolated by a rapid two-stage method involving affinity chromatography of C1 on IgG-Sepharose and isolation of subcomponent C1s by ion-exchange chromatography on DEAE-Sephacel. Single-chain C1s proenzyme was activated to two-chain C1s with self-activated C1r. After reduction and S-carboxamidomethylation the heavy chain of C1s (Mr 57 000) was isolated by ion exchange chromatography on DEAE-Sephacel. Cleavage of C1s heavy chain with CNBr yielded five fragments whose N-terminal sequences were determined. The alignment of the fragments within the heavy chain was established by tryptic peptides containing methionine. C1s heavy chain comprises about 470 amino acid residues and 42% of its sequence was determined. An intrachain sequence homology and a homology to the alpha 2 chain of human haptoglobin were identified. The C-terminal CNBr fragment comprising 44 amino acid residues was completely sequenced. From BNPS-skatole cleavage of reduced and alkylated C1s proenzyme a fragment was isolated which overlaps the C1s heavy and light chain parts and which contains the peptide bond cleaved during activation. The results show that this is an Arg-Ile bond and that under standard conditions of activation no peptide material is liberated from this portion of the molecule. The sequence data and homology to two-chain serine proteases indicate a single interchain disulfide bond in C1s.