Role of protein kinase C-η in cigarette smoke extract-induced apoptosis in MRC-5-cells.

Cigarette smoke (CS) is a major risk factor for emphysema, which causes cell death in structural cells of the lung by mechanisms that are still not completely understood. We demonstrated previously that CS extract (CSE) induces caspase activation in MRC-5 human lung fibroblasts, activated protein kinase C-η (PKC-η), and translocated PKC-η from the cytosol to the membrane. The objective of this study was to investigate the involvement of PKC-η activation in a CSE-induced extrinsic apoptotic pathway. We determined that CSE increases expression of caspase 3 and 8 cleavage in MRC-5 cells and overexpression of PKC-η significantly increased expression of caspase 3 and 8 cleavage compared with control LacZ-infected cells. In contrast, dominant negative (dn) PKC-η inhibited apoptosis in MRC-5 cells exposed to CSE and decreased expression of caspase 3 and 8 compared with control cells. Exposure to 10% CSE for >8 h significantly increased lactate dehydrogenase release in PKC-η-infected cells compared with LacZ-infected cells. Additionally, PKC-η-infected cells had an increased number of Hoechst 33342 stained nuclei compared with LacZ-infected cells, while dn PKC-η-infected cells exhibited fewer morphological changes than LacZ-infected cells under phase-contrast microscopy. In conclusion, PKC-η activation plays a pro-apoptotic role in CSE-induced extrinsic apoptotic pathway in MRC-5 cells. These results suggest that modulation of PKC-η may be a useful tool for regulating the extrinsic apoptosis of MRC-5 cells by CSE and may have therapeutic potential in the treatment of CS-induced lung injury.

Evaluation of the antioxidant effect of ganghwayakssuk (Artemisia princeps Pamp.) extract alone and in combination with ascorbic acid in raw chicken patties.

We investigated the inhibition of lipid oxidation of raw chicken patties by the antioxidants ascorbic acid (Aa), ganghwayakssuk extracts (GE), and their combination (Aa + GE). All antioxidant combinations were effective at delaying lipid oxidation compared with the control or Aa. A combination of Aa + GE (0.05% Aa + 0.2% GE) was the most effective for delaying lipid oxidation (TBA reactive substances, conjugated dienes, and peroxide formation). The color values of all samples were significantly affected by adding GE. Additionally, the redness, color difference, and hue values of all treatments, except for Aa, were lower than those of the control as the amount of GE increased. The total viable bacterial counts of samples with GE 0.2 and Aa + GE 0.2 were significantly affected during storage (P < 0.05). The results suggest that adding an antioxidant combination reduced the oxidative stress and microbial growth of raw chicken patties stored for 12 d under normal refrigeration temperature, which may extend the shelf life of chicken patties.

The Eyes Absent phosphatase-transactivator proteins promote proliferation, transformation, migration, and invasion of tumor cells.

The Eyes Absent (EYA) proteins combine transactivation, tyrosine phosphatase, and threonine phosphatase activities in their function as part of a conserved regulatory cascade involved in embryonic organ development. EYA tyrosine phosphatase activity contributes to fly eye development, and vertebrate EYA is involved in promoting DNA damage repair subsequent to genotoxic stress. EYAs are known to be expressed at elevated levels in ovarian and breast cancers. Here, we show that the tyrosine phosphatase activity of the EYAs promotes tumor cell migration, invasion, and transformation. These cellular effects are accompanied by alterations of the actin cytoskeleton and increased levels of active Rac and Cdc42. The invasiveness conferred by EYA is reflected in vivo by inhibition of metastasis seen when EYA3 expression is silenced in the invasive breast cancer cell line MDA-MB-231. Together, our data directly associate the tyrosine phosphatase activity of the EYAs with the oncogenesis-associated cellular properties of motility and invasiveness.

A new HIF-1 alpha variant induced by zinc ion suppresses HIF-1-mediated hypoxic responses.

The expressions of hypoxia-inducible genes are upregulated by hypoxia-inducible factor 1 (HIF-1), which is a heterodimer of HIF-1alpha and HIF-1beta/ARNT (aryl hydrocarbon receptor nuclear transporter). Under hypoxic conditions, HIF-1alpha becomes stabilized and both HIF-1alpha and ARNT are translocated into the nucleus and codimerized, binding to the HIF-1 consensus sequence and transactivating hypoxia-inducible genes. Other than hypoxia, cobalt and nickel, which can substitute for iron in the ferroprotein, induce the stabilization of HIF-1alpha and the activation of HIF-1. We found previously that, although zinc, another example of a metal substitute for iron, stabilized HIF-1alpha, it suppressed the formation of HIF-1 by blocking the nuclear translocation of ARNT. Here, we identify a new spliced variant of human HIF-1alpha that is induced by zinc. The isoform lacks the 12th exon, which produced a frame-shift and gave a shorter form of HIF-1alpha (557 amino acids), designated HIF-1alphaZ (HIF-1alpha induced by Zn). This moiety was found to inhibit HIF-1 activity and reduce mRNA expressions of the hypoxia-inducible genes. It blocked the nuclear translocation of ARNT but not that of endogenous HIF-1alpha, and was associated with ARNT in the cytosol. These results suggest that HIF-1alphaZ functions as a dominant-negative isoform of HIF-1 by sequestering ARNT in the cytosol. In addition, the generation of HIF-1alphaZ seems to be responsible for the inhibitory effects of the zinc ion on HIF-1-mediated hypoxic responses, because the expressed HIF-1alphaZ behaved in the same manner as zinc in terms of inhibited HIF-1 activity and ARNT translocation.

Inhibitory effect of YC-1 on the hypoxic induction of erythropoietin and vascular endothelial growth factor in Hep3B cells.

YC-1 is a newly developed agent that inhibits platelet aggregation and vascular contraction. Although its effects are independent of nitric oxide (NO), it mimics some of the biological actions of NO. For example, it stimulates soluble guanylate cyclase (sGC) and increases intracellular cGMP concentration. Here, we tested the possibility that YC-1 inhibits hypoxia-inducible factor (HIF)-1-mediated hypoxic responses, as does NO. Hep3B cells were used during the course of this work to observe hypoxic induction of erythropoietin (EPO) and vascular endothelial growth factor (VEGF), and the effects of YC-1 were compared with those of a NO donor, sodium nitropurruside (SNP). In hypoxic cells, YC-1 blocked the induction of EPO and VEGF mRNAs, and inhibited the DNA-binding activity of HIF-1. It suppressed the hypoxic accumulation of HIF-1alpha, but not its mRNA level. It also reduced HIF-1alpha accumulation induced by cobalt and desferrioxamine. Treatment with antioxidants did not recover the HIF-1alpha suppressed by YC-1. We examined whether these effects of YC-1 are related to the sGC/cGMP signal transduction system. Two sGC inhibitors examined failed to block the effects of YC-1, and 8-bromo-cGMP did not mimic actions of YC-1. The effects of YC-1 on the hypoxic responses were comparable with those of SNP. These results suggest that YC-1 and SNP suppressed the hypoxic responses by post-translationally inhibiting HIF-1alpha accumulation. The YC-1 effect may be linked with the metal-related oxygen sensing pathway, and is not due to the stimulation of sGC. This observation implies that the inhibitory effects of YC-1 on hypoxic responses can be developed to suppress EPO-overproduction by tumor cells and tumor angiogenesis.

Effect of short-term ethanol on the proliferative response of Swiss 3T3 cells to mitogenic growth factors.

Both adaptive and deleterious responses of cells to ethanol are likely triggered by short-term interactions of the cells with ethanol. Many studies have demonstrated the direct effect of ethanol on growth factor-stimulated cell proliferation. Using Swiss 3T3 cells whose growth was inhibited by ethanol in a concentration-dependent manner, we further investigated the molecular mechanisms of acute ethanol treatment by examining its effect on EGF- and PDGF-mediated cellular signaling systems for the mitogenic function. Tyrosine autophosphorylation of the growth factor receptors was partially prevented by ethanol in intact cells. When ethanol was included before or after EGF stimulation, no effect on the receptor signaling was observed. Here we also report that ethanol inhibits activation of ERK induced by both EGF and PDGF. EGF-induced JNK activation was reduced but PDGF-induced rapid JNK activation was delayed by the addition of ethanol. The balance between its inhibitory and stimulatory effect on the signaling molecules might determine the rate of cell growth.

Senescence-like changes induced by hydroxyurea in human diploid fibroblasts.

Hydroxyurea was found to inhibit the growth of human diploid fibroblasts, which resulted in senescence-like changes both in morphology and replicative potential similar to the replicative senescence. SA-beta-gal activity, a typical characteristic of the replicative senescence was also induced through a long-term treatment of the presenescent cells with 400-800 microgM of hydroxyurea for about 3 weeks. In addition, we determined the levels of cyclin-dependent kinase inhibitors, p21(Waf1) and p16(INK4a), and the p53 tumor suppressor in order to monitor its effect on cell cycle and stress responses. We observed a great induction of both p53 and p21(Waf1), but not of p16(INK4a) in the premature senescent cells. UV-irradiation of the premature senescent cells showed a decreased level of DNA fragmentation presumably ascribed to the reduced activation of stress-activated protein kinases. These results suggest that a chronic hydroxyurea treatment induces the cellular senescence in association with the induction of p53 and p21(Waf1).

Reduction of UV-induced cell death in the human senescent fibroblasts.

We studied mechanisms by which senescent cells acquire resistance to UV-induced cellular insults. Human primary foreskin fibroblast culture was used since it undergoes cellular senescence in vitro after a limited number of passages. Senescence was induced by a brief treatment of the early passage cells with 100 microM of H2O2 for 1 h, and subsequent culture for 3 weeks. Hydrogen peroxide-treated cells showed an enhancement of senescence-associated beta-galactosidase activity. In the senescent cells, DNA fragmentation in response to UV-irradiation was found to decrease significantly compared with that in the young cells. The SAPK/JNK activation by UV irradiation was reduced in both non-treated senescent cells and the hydrogen peroxide-induced senescent cells, suggesting that a reduced DNA fragmentation by UV-irradiation in the senescent cells is closely related to the decreased SAPK/JNK activity. Since a cell cycle inhibitor, p21Waf1, has been implicated in protecting cells against apoptotic cell death, we determined p21Waf1 to assess whether its elevation has any impact on the reduction of UV-induced activation of SAPK/JNK in the senescent cells. The expression of p21Waf1 increased in both the nontreated and the hydrogen peroxide-treated senescent cells. Our study also revealed that the blockage of SAPK/JNK activation in the senescent cells was closely related to the increased level of p21Waf1. Our observation might provide clues about molecular mechanism of resistance to DNA fragmentation and the consequent cell death by UV-irradiation.

Inhibition of glycine N-methyltransferase by 5-methyltetrahydrofolate pentaglutamate.

Glycine N-methyltransferase (EC 2.1.1.20) catalyzes the methylation of glycine by S-adenosylmethionine to form sarcosine and S-adenosylhomocysteine. The enzyme was previously shown to be abundant in both the liver and pancreas of the rat, to consist of four identical monomers, and to contain tightly bound folate polyglutamates in vivo. We now report that the inhibition of glycine N-methyltransferase by (6S)-5-CH(3)-H(4)PteGlu(5) is noncompetitive with regard to both S-adenosylmethionine and glycine. The enzyme exhibits strong positive cooperativity with respect to S-adenosylmethionine. Cooperativity increases with increasing concentrations of 5-CH(3)-H(4)PteGlu(5) and is greater at physiological pH than at pH 9.0, the pH optimum. Under the same conditions, cooperativity is much greater for the pancreatic form of the enzyme. The V(max) for the liver form of the enzyme is approximately twice that of the pancreatic enzyme, while K(m) values for each substrate are similar in the liver and pancreatic enzymes. For the liver enzyme, at pH 7.0 half-maximal inhibition is seen at a concentration of about 0.2 microM (6S)-5-CH(3)-H(4)PteGlu(5), while at pH 9.0 this value is increased to about 1 microM. For the liver form of the enzyme, 50% inhibition with respect to S-adenosylmethionine at pH 7.4 occurs at about 0.27 microM. The dissociation constant, K(s), obtained from binding data at pH 7.4 is 0.095. About 1 mol of (6S)-5-CH(3)-H(4)PteGlu(5) was bound per tetramer at pH 7.0, and 1.6 mol were bound at pH 9.0. The degree of binding and inhibition were closely parallel at each pH. At equal concentrations of (6R,6S)- and (6S)-5-CH(3)-H(4)PteGlu(5), the natural (6S) form was about twice as inhibitory. These studies indicate that glycine N-methyltransferase is a highly allosteric enzyme, which is consistent with its role as a regulator of methyl group metabolism in both the liver and the pancreas.

Aging process is accompanied by increase of transglutaminase C.

Crosslinking has been suggested as one of the mechanisms involved in the aging process. Among the various random or enzyme-mediated crosslinking reactions, transglutaminase (TGase)-catalyzed crosslinking activity has been proposed for its possible involvement in cell proliferation, differentiation, carcinogenesis, programmed death, and aging. Moreover, recent findings of TGase C as a putative signal transducer and cell cycle regulator has renewed interest in the study of TGase C in relation to aging phenomena. The ubiquitous presence of TGase C compared to the organ-specific localization of other types of TGases has attracted special attention as a cellular aging device. In the present investigation for in vitro studies, we have compared the pattern of TGase C in young and old human red blood cells, separated by density differentiation, and in early and late-passage or hydrogen peroxide-treated human primary fibroblasts. For in vivo study, we monitored the age-dependent changes of TGase C in the liver and brain tissues of 4, 12, 18, and 24-month-old Sprague-Dawley rats. We obtained evidence that both the activity and protein levels of TGase C were high in old RBC and late-passage or hydrogen peroxide-treated fibroblasts. Similar findings were seen in liver and brain tissue such as age-dependent increases in TGase activity and protein level in an organ-specific pattern. These data suggest that TGase C might play an active role in the cellular process with age.

The effect of pyrimidine nucleosides on adenosine-induced apoptosis in HL-60 cells.

Among several nucleosides, adenosine is the only one to induce typical apoptotic cell death in human promyelocytic leukemia HL-60 cells. Intracellularly transported adenosine seemed to be required for the induction of apoptosis, since dipyridamole, which inhibits the transport of adenosine, strongly suppressed apoptosis, and 8-phenyltheophylline, a receptor antagonist, did not affect the adenosine-induced effect. The viability of adenosine-treated HL-60 cells was partially recovered by supplementation with a pyrimidine nucleoside, uridine or thymidine. Cytidine or deoxycytidine had no effect on the growth and survival of adenosine-treated cells, while uridine or thymidine inhibited adenosine-induced intracellular DNA fragmentation. These results suggest that the quantitative adjustment of purine and pyrimidine nucleosides might play an important role in the adenosine-induced apoptosis of HL-60 cells. The reduction of c-Myc expression in adenosine-treated cells was prevented by uridine or thymidine. These observations suggest that the expression of c-Myc might be related to an intracellular sensing system for the quantitative balance of nucleosides or nucleotides.

Phospholipase D activity in L1210 cells: a model for oleate-activated phospholipase D in intact mammalian cells.

Phospholipase D (PLD) in lymphocytic mouse leukemic L1210 cells has been found to be activated by oleate both in vitro and in intact cells. The PLD activity was measured by phosphatidylethanol produced from radiolabeled phosphatidylcholine or myristic acid in the presence of ethanol. This oleate-activated PLD was further characterized in intact cells and compared with that in HL60 cells. Unlike PLD in HL60 cells, the PLD in L1210 cells was activated by unsaturated fatty acids, stimulated by melittin, insensitive to guanosine 5'-(3-O-thio)triphosphate (GTP gamma S), ADP-ribosylation factor (ARF) and phosphatidylinositol 4,5-bisphosphate (PIP2), independent of phorbol 12-myristate 13-acetate (PMA) and staurosporine, and inhibited by pervanadate. These observations indicate that the PLD present in L1210 cells is distinct from that in HL60 cells. Key PLD properties of L1210 cells such as insensitivity to GTP gamma S, ARF, PIP2, or PMA were in good agreement with currently known in vitro properties of the oleate-activated PLD found in mammalian sources. Therefore, the L1210 cells could be used as an intact-cell source for an oleate-activated PLD.

Dissociation of tyrosine phosphorylation and activation of phosphoinositide phospholipase C induced by the protein kinase C inhibitor Ro-31-8220 in Swiss 3T3 cells treated with platelet-derived growth factor.

Platelet-derived growth factor (PDGF) stimulates the hydrolysis of phosphatidylinositol 4,5-bisphosphate (Ptd InsP2) via phospholipase C-gamma1 (PLC-gamma1) in Swiss 3T3 cells. Treatment of cells with the protein kinase C (PKC) inhibitor Ro-31-8220 greatly decreased PDGF-induced tyrosine phosphorylation of PLC-gamma1, but paradoxically enhanced the production of inositol phosphates (InsPs). The inhibitor also caused an increase of PDGF receptor tyrosine phosphorylation at later times. The changes in phosphorylation of the receptor were correlated with alterations in PLC-gamma1 translocation to the particulate fraction. Thus, although activation of PLC-gamma1 was associated with phosphorylation of the receptor and translocation of the enzyme to the particulate fraction, it was dissociated from its tyrosine phosphorylation. A non-receptor-associated, cytosolic tyrosine kinase also was found to phosphorylate PLC-gamma1 in a PDGF-dependent manner, but was not inhibited by Ro-31-8220 in vitro. PKC depletion by phorbol ester treatment decreased the tyrosine phosphorylation of PLC-gamma1 induced by PDGF and slowed the translocation of PLC-gamma1, but Ro-31-8220 produced further effects. The effect of Ro-31-8220 to enhance the production of InsPs could not be attributed to inhibition of PKC since InsPs production with PDGF was decreased in PKC-depleted cells and a stimulatory effect of the inhibitor was still evident. Interestingly, Ro-31-8220 decreased the radioactivity in phosphatidylinositol and increased that in phosphatidylinositol 4-phosphate and PtdInsP2 in cells labeled with myo[3H]inositol. The increased synthesis of PtdInsP2 could contribute to the increased production of InsPs induced by Ro-31-8220. In summary, these results support the conclusion that the activation of PLC-gamma1 in response to PDGF requires autophosphorylation of the receptor and membrane association of PLC-gamma1, but not phosphorylation of the enzyme. Furthermore, the effects of Ro-31-8220 on the tyrosine phosphorylation and activity of PLC-gamma1, and on PtdInsP2 synthesis cannot be attributed to inhibition of PKC.

Stimulation of phospholipase D by epidermal growth factor requires protein kinase C activation in Swiss 3T3 cells.

The proposal that epidermal growth factor (EGF) activates phospholipase D (PLD) by a mechanism(s) not involving phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) hydrolysis was examined in Swiss 3T3 fibroblasts. EGF, basic fibroblast growth factor (bFGF), bombesin, and platelet-derived growth factor (PDGF) activated PLD as measured by transphosphatidylation of butanol to phosphatidylbutanol. The increase in inositol phosphates induced by bFGF, EGF, or bombesin was significantly enhanced by Ro-31-8220, an inhibitor of protein kinase C (PKC), suggesting that PtdIns(4,5)P2-hydrolyzing phospholipase is coupled to the receptors for these agonists but that the response is down-regulated by PKC. Activation of PLD by EGF was inhibited dose dependently by the PKC inhibitors bis-indolylmaleimide and Ro-31-8220, which also inhibited the effects of bFGF, bombesin, and PDGF. Down-regulation of PKC by prolonged treatment with 4 beta-phorbol 12-myristate 13-acetate also abolished EGF- and PDGF-stimulated phosphatidylbutanol formation. EGF and bombesin induced biphasic translocations of PKC delta and epsilon to the membrane that were detectable at 15 s. In the presence of Ro-31-8220, translocation of PKC alpha became evident, and membrane association of the delta- and epsilon-isozymes was enhanced and/or sustained in response to the two agonists. The inhibitor also enhanced EGF-stimulated [3H]diacylglycerol formation in cells preincubated with [3H]arachidonic acid, which labeled predominantly phosphatidylinositol, but inhibited [3H]diacylglycerol production in cells preincubated with [3H]myristic acid, which labeled mainly phosphatidylcholine. These data support the conclusion that EGF can stimulate diacylglycerol formation from PtdIns(4,5)P2 and that PKC performs the dual role of down-regulating this response as well as mediating phosphatidylcholine hydrolysis. In summary, all of the results of the study indicate that PLD activation by EGF is downstream of PtdIns(4,5)P2-hydrolyzing phospholipase and is dependent upon subsequent PKC activation.

Activation of phospholipase C-gamma is necessary for stimulation of phospholipase D by platelet-derived growth factor.

Platelet-derived growth factor (PDGF) stimulates phosphatidylcholine hydrolysis via phospholipase D (PLD) in several tissues. To determine whether PLD activation is dependent on phosphoinositide hydrolysis by phospholipase C (PLC), we measured the formation of phosphatidylbutanol (PtdBut), in TRMP cells overexpressing wild type or various mutant PDGF receptors. Both PLC and PLD were stimulated by PDGF in cells expressing wild type receptors whereas they were not in cells expressing kinase-deficient (R634) receptors. These data indicate that tyrosine phosphorylation is required for activation of both PLC and PLD. Mutation of Tyr-1021 of the PDGF receptor to Phe caused loss of PDGF stimulation of both PLC and PLD. On the other hand, a mutant PDGF receptor that was able to bind PLC gamma 1 but not other signaling proteins (including the Ras GTPase-activating protein, phosphatidylinositol 3-kinase, and a SH2-containing phosphotyrosine phosphatase (Syp)) restored the stimulatory effect of PDGF on PLC and PLD. Furthermore, receptors in which association with the GTPase-activating protein, phosphatidylinositol 3-kinase, or Syp was individually restored were unable to mediate PDGF stimulation of PLC or PLD. These data indicate that these other signal transduction proteins are not involved in the activation of PLD by PDGF. Treatment of the cells with the protein kinase C inhibitor, Ro-31-8220, and depletion of cellular protein kinase C by pretreatment with 4 beta-phorbol 12-myristate 13-acetate resulted in loss of PLD activation by PDGF indicating a PKC-dependent mechanism. In summary, these results indicate that activation of PLC gamma 1 and protein kinase C are necessary for the stimulation of PLD by PDGF and provide no evidence for alternative mechanisms.

Lysophosphatidic acid activation of phosphatidylcholine-hydrolysing phospholipase D and actin polymerization by a pertussis toxin-sensitive mechanism.

Incubation of IIC9 fibroblasts with lysophosphatidic acid (LPA) induced an increase in the amount of filamentous actin (F-actin), which was concentration-dependent with a maximal effect at 100 ng/ml. Phosphatidic acid (PA) also produced a concentration-dependent increase of F-actin, but it was less potent than LPA. The LPA-induced increase in F-actin was rapid and sustained for at least 60 min. LPA rapidly increased the levels of PA and choline, with maximal increases at 5 min and 30 s respectively. LPA also caused a monophasic increase in diacylglycerol (DAG) which lagged behind the increases in PA and choline. LPA stimulated phosphatidylbutanol formation in the presence of butanol and produced a small increase in inositol phosphates that was much less than that induced by alpha-thrombin. Pretreatment of cells with pertussis toxin (PTX) caused greater than 50% inhibition of the LPA-stimulated increases in PA, DAG and choline. PTX increased the LPA concentration required to induce half-maximal actin polymerization by about 10-fold. PTX caused a similar shift in the dose-response curve for LPA-induced PA formation. These results suggest that LPA induces an increase in PA by activating a phosphatidylcholine-hydrolysing phospholipase D via a PTX-sensitive G-protein and that the increase in PA is involved in the activation of actin polymerization.

Tissue distribution of glycine N-methyltransferase, a major folate-binding protein of liver.

Glycine N-methyltransferase (GNMT; S-adenosyl-L-methionine:glycine N-methyltransferase, EC 2.1.1.20) is a major protein in rat liver that binds 5-methyltetrahydrofolate polyglutamate in vivo. This enzyme is believed to function in the regulation of the availability of S-adenosylmethionine, the primary donor of methyl groups in the body. The distribution of GNMT in a variety of rat tissues was examined immunohistochemically. In liver, GNMT was most abundant in the periportal region, whereas in kidney it was seen primarily in the proximal convoluted tubules. In pancreas, GNMT was abundant, principally in the exocrine tissue. GNMT was present in the striated duct cells of the submaxillary gland. In the jejunum, GNMT was found in the epithelial cells of the villi. Close examination of the liver indicated GNMT in the nucleus; this site was confirmed by purification of the nuclei and measurement of enzyme activity. The location of GNMT in the liver and kidney suggests that this enzyme plays a role in gluconeogenesis, while its presence in the exocrine cells suggests it may also be a factor in secretion.

Purification and properties of pancreatic glycine N-methyltransferase.

Glycine N-methyltransferase (GNMT) regulates the ratio of S-adenosylmethionine to S-adenosylhomocysteine. It is very abundant in liver cytosol and earlier studies have shown it to be present in high concentrations in the pancreas. We have previously reported that liver GNMT is allosterically inhibited by 5-methyltetrahydrofolate pentaglutamate (5-CH3-H4PteGlu5), and proposed that this represents a metabolic control mechanism which links the de novo synthesis of methyl groups to the methylating ability of the liver. We now report that pancreatic GNMT also contains bound folate in vivo. Purified pancreatic GNMT is inhibited by reduced folate polyglutamates in vitro. The KI for the synthetic (R,S)5-CH3-H4PteGlu5 is 2.4 x 10(-7) M. The natural (S) form of 5-CH3-H4PteGlu5 is tightly bound and has a Kd of 1.3 x 10(-7) M. One mole is bound per enzyme tetramer. These studies suggest that GNMT is important in the regulation of methyl group metabolism in the pancreas as well as in the liver.