A small G-protein involved in phosphatidylinositol-4-phosphate kinase activation.

A guanine nucleotide binding protein tentatively designated Gs(PIPK) which activates purified phosphatidylinositol-4-phosphate kinase in vitro, has been partially purified from rat liver membranes and identified as a small G-protein with a molecular mass between 20 and 25 kDa.

Purification and partial characterization of phosphatidylinositol-4-phosphate kinase from rat liver plasma membranes. Further evidence for a stimulatory G-protein.

Phosphatidylinositol 4-phosphate (PIP) kinase (E.C. 2.7.1.68) has been purified about 1200-fold from rat liver plasma membranes, taking advantage of affinity chromatography on quercetin-Sepharose as a novel step. The purified PIP kinase showed no contamination by the following enzyme activities: phosphatidylinositol (PI) kinase (EC 2.7.1.67), protein kinase C (EC 2.7.1.-), diacylglycerol kinase (EC 2.7.1.-), phospholipase C (EC 3.1.4.11), protein-tyrosine kinase (EC 2.7.1.112), alkaline phosphatase (EC 3.1.3.1), triphosphoinositide phosphomonoesterase (EC 3.1.3.36), adenylate kinase (EC 2.7.4.3) and cAMP-dependent protein kinase (EC 2.7.1.37). The liver membrane enzyme requires high Mg2+ concentrations with a KM value of 10 mM. Ca2+ or Mn2+ could replace Mg2+ to a certain, though small, extent. Apparent KM values with respect to PIP and ATP were 10 and 65 microM, respectively. GTP was slightly utilized by the kinase as phosphate donor while CTP was not. Quercetin inhibited the enzyme with Ki = 34 microM. Extending our previous observations (Urumow, T. and Wieland, O.H. (1986) FEBS Lett. 207, 253-257 and Urumow, T. and Wieland, O.H. (1988) Biochim. Biophys. Acta 972, 232-238) [gamma S]pppG still stimulated the PIP kinase in extracts of solubilized liver membranes. 20-40% (NH4)2SO4 precipitation of the membrane extracts yielded a fraction that contained the bulk of enzyme activity but did not respond to stimulation by [gamma S]pppG any longer. This was restored by recombination with a protein fraction collected at 40-70% (NH4)2SO4 saturation, presumably containing a GTP binding protein and/or some other factor separated from the PIP kinase. In the reconstituted system [gamma S]pppG stimulated PIP kinase in a concentration dependent manner with maximal activation at 5 microM. This effect was not mimicked by [gamma S]pppA and was blocked by [beta S]ppG. These results strongly support our view that in liver membranes PIP kinase is regulated by a G-protein.

Protein glycation: measurement and clinical relevance.

This review of protein glycation deals with the biochemical background of glycated blood proteins, their methods of determination and their clinical significance. General reaction principles for determination of glycated proteins are discussed with special emphasis on the determination of glycated serum proteins in the clinical laboratory. Binding methods like boronate affinity, immunoassay, phenylhydrazine binding and ion exchange chromatography leave the analyte intact, whereas chemical methods like strong or mild hydrolysis or reduction in alkaline medium (fructosamine assay) results in destruction of the glycated protein. As most reactions are nonstoichiometric (except ion exchange chromatography and periodate oxidation), varying results are obtained from laboratory to laboratory. Up to now boronate affinity chromatography, mild hydrolysis yielding hydroxymethylfurfural and the fructosamine assay have been mostly used for determination of glycated serum proteins. The fructosamine assay appears to be most practical, because it is quick, economic and precise, but it suffers from unspecific side reactions. Although other methods like immunoassays or boronate ester formation in solution appear promising, there is currently no commercially available assay for the economic, precise and accurate determination of glycated serum protein. The clinical relevance of glycated serum protein determination is difficult to evaluate because the assays are based on different reaction principles and hence yield variable results. Nevertheless, the following conclusion may be drawn from the reports now available. i) The possibility that glycated serum proteins may discriminate better than glycated haemoglobin between "normal" and "diabetic" is still controversial. ii) Glycated serum proteins are formed faster than glycated haemoglobin, reflecting the changes in glycaemia for shorter periods of time (medium-term control). iii) It has not been yet established, using large cohorts, whether the glycated serum proteins allow the detection or exclusion of diabetes. iv) Determination of glycated serum proteins should not be considered as a substitute for the determination of glycated haemoglobin, but rather as a complementary determination, leading to the improved laboratory control of the diabetic patient.

Evidence for a cholera-toxin-sensitive G-protein involved in the regulation of phosphatidylinositol 4-phosphate kinase of rat liver membranes.

Studies on the phosphorylation of inositol phospholipids of rat liver membranes have shown that [gamma S]pppG stimulates 32P incorporation from [gamma-32P]ATP into PI and PIP. This effect appeared specific for stable GTP analogues and could not be reproduced by other compounds. ADP-ribosylation of the membranes with cholera toxin resulted in a large decrease of PIP2 without changes in the level of PIP. Since an activation of phospholipase C can be ruled out, the lowering of PIP2 is explained on the basis of an inhibition of PIP kinase (EC 2.7.1.68). From these results it appears that a novel cholera-toxin-sensitive G-protein is involved in the regulation of PIP kinase.

Separation of the protein-tyrosine kinase and phosphatidylinositol kinase activities of the human placental insulin receptor.

On immunoprecipitation using a specific antiphosphotyrosine antibody, phosphatidylinositol kinase (EC 2.7.1.67) activity was separated from the protein-tyrosine kinase (EC 2.7.1.112) activity of the wheat germ agglutinin (WGA) -purified insulin receptor from human placenta. This clearly indicates that protein-tyrosine kinase and phosphatidylinositol kinase activity do not reside on the same polypeptide chain as previously has been suggested. Quantitatively, the fraction of phosphatidylinositol kinase that was bound to WGA sepharose and eluted together with the insulin receptor amounted to 2% of the Triton X-100 soluble phosphatidylinositol kinase. The apparent Km values of the bound and unbound phosphatidylinositol kinase with respect to PI and ATP were very similar (0.4 and 0.3 mmol/l and 8 and 7 mumol/l, respectively) suggesting that the WGA-bound phosphatidylinositol kinase is not a different enzyme, but rather represents a small portion of the bulk Triton X-100-soluble phosphatidylinositol kinase that is bound to the lectin tightly associated with the insulin receptor. The synthetic polymer (Glu80Tyr20)n, a model substrate of the insulin receptor tyrosine kinase, at 0.5 mmol/l, inhibited phosphatidylinositol kinase of WGA-purified insulin receptor by 70-90%. This inhibition was not overcome by increasing the concentrations of ATP or PI as one would expect if a functional interrelationship of the protein-tyrosine kinase and the phosphatidylinositol kinase would exist.

Enzymatic assay of myo-inositol in serum.

An enzymatic spectrophotometric method for determination of myo-inositol in serum is described. The method is sensitive and rapid to perform without special equipment. Linearity between the amount of myo-inositol and absorbance was obtained in the range of 0.5 to 3 nmol myo-inositol per assay. The amounts of myo-inositol determined in sera from apparently healthy subjects agree well with gas chromatographic data.

An Mr 180,000 protein is an endogenous substrate for the insulin-receptor-associated tyrosine kinase in human placenta.

The beta-subunit of the insulin receptor contains a tyrosine-specific protein kinase. Insulin binding activates this kinase and causes phosphorylation of the beta-subunit of the insulin receptor. It is believed that phosphorylation of other proteins might transmit the insulin signal from the receptor to the cell. In the present study we used a polyclonal anti-phosphotyrosine antibody to detect other proteins that become tyrosine phosphorylated upon insulin stimulation. Glycoproteins from human placenta membranes were enriched by wheat germ agglutinin chromatography and phosphorylation was studied with [gamma-32P]ATP and insulin in vitro. Phosphorylated proteins were immunoprecipitated by antibodies against the insulin receptor and by serum containing the anti-phosphotyrosine antibody. Beside the insulin-stimulated phosphorylation of the 95 kDa beta-subunit of the insulin receptor, an insulin-stimulated phosphorylation of a 180 kDa protein was found. The phosphorylation of both proteins occurred only on tyrosine residues. Insulin increased 32P incorporation into the 180 kDa band 2.7-fold (S.E.M. +/- 0.3, n = 5). The 180 kDa protein was not precipitated by antibodies against the insulin receptor. H.p.l.c. chromatograms of tryptic fragments of the phosphorylated 180 kDa protein and of the beta-subunit of the insulin receptor revealed different patterns for both proteins. Insulin-stimulated phosphorylation of the 180 kDa protein was also detectable in unfractionated detergent-solubilized membranes. The phosphorylation of the 180 kDa protein was stimulated by insulin with the same dose-response curve as the phosphorylation of the beta-subunit, suggesting that this protein might be another endogenous substrate of the insulin receptor kinase.

Kinetic analysis of glycation as a tool for assessing the half-life of proteins.

Glycation of proteins, a common postribosomal modification, proceeds via Amadori rearrangement to yield a stable ketoamine linkage of glucose with the protein. Kinetic analysis of the reaction shows that the amount of glycation at steady state is proportional to the glucose concentration, to protein half-life and to the rate of glycation. Thus, when the rate of glycation is determined in vitro and the extent of glycation of a given protein isolated from euglycemic subjects is measured, the half-life may be calculated. As the in vivo situation may not be simulated accurately in vitro, the calculated values may be considered as approximation. When the calculated values were compared with values reported in the literature fairly good agreement was found except for hemoglobin. Studies on stability of glycated albumin show that ketoamine decreases by about 20% when incubated under physiological conditions for 20 days. The method described by us is especially valuable when turnover of proteins in normal and pathophysiological states are compared. The half-life of plasma low-density lipoprotein is longer in patients with hypothyroidism or a high plasma low-density lipoprotein level than in normal subjects. Extending our studies to tissue proteins we did not find a significant increase in half-life of tendon collagen with age. Basement membrane collagen turnover is faster in diabetic patients in bad metabolic control. Thus, the procedure using fructosylamine as endogenous label of protein offers a method of great potential to study the turnover of human body proteins.

Stimulation of phosphatidylinositol 4-phosphate phosphorylation in human placenta membranes by GTP gamma S.

In human placenta membranes the rate limiting enzyme for PIP2 formation from PI is PIP kinase. GTP gamma S is shown to activate PIP kinase by increasing Vmax of the enzyme. It is suggested that a guanine nucleotide regulatory protein is involved in the activation of PIP kinase although coupling with a specific receptor is not yet known. Since PIP2 is the preferred substrate of phospholipase C, the possibility exists that an increase of PIP2 due to activation of PIP kinase leads to an enhancement of phospholipase C activity and hence to an increased production of IP3 and DAG.

Evidence that (a) serine specific protein kinase(s) different from protein kinase C is responsible for the insulin-stimulated actin phosphorylation by placental membrane.

Partially purified phospholipid- and Ca2+-dependent protein kinase C from human placenta catalyzes the Mg-ATP-dependent phosphorylation of serine residues of purified rabbit muscle actin. Two tryptic [32P]-phosphopeptides were found on HPLC separation. Confirming the previous report by Machicao and Wieland [(1985) Curr. Top. Cell. Regul. 27, 95-105], actin is phosphorylated at serine residues by human placental membranes, and this is stimulated by insulin. In the absence of insulin trypsin treatment yielded eight [32P]phosphopeptides, two of which coincided with the ones due to protein kinase C. Insulin led to the appearance of three new [32P]phosphopeptides. These results suggest that insulin stimulates (a) serine protein kinase(s) which, like protein kinase C, is present in placental membranes.

Endothelial plasma membrane is a glucocorticoid-regulated barrier for the uptake of glucose into the cell.

The effect of glucose concentrations and hormones on glucose consumption, lactate, pyruvate, sorbitol and fructose formation of porcine aortic endothelial cells and human umbilical vein endothelial cells has been investigated. Endothelial cells have a high glycolytic activity which is saturated far below physiologic blood glucose levels (KM apparent less than 1 mmol/l). Glucocorticoids reduce glucose catabolism as a function of their concentration. Insulin, adrenaline, triiodothyronine and glucagon do not influence glucose consumption. Studies with the non-metabolizable analogue 3-O-methyl-D-glucose revealed that glucocorticoids slow down glucose transport into the endothelial cell. The passage of glucose through the cell membrane is the rate-limiting step of glucose utilization. Consequently, the intracellular glucose level is independent of the ambient glucose concentration and endothelial cells do not accumulate sorbitol under hyperglycaemic conditions since the affinity of aldose reductase for glucose is low.

Limited nonenzymatic glucosylation of low-density lipoprotein does not alter its catabolism in tissue culture.

This study examines the effects of various degrees of chemical modification of low-density lipoprotein (LDL) on its catabolism by various cell types. Moderate glucosylation of LDL does not alter its interaction with the high-affinity receptor present on human fibroblasts at concentration of 5-2000 micrograms LDL-cholesterol/ml. Only heavily glucosylated LDL (more than 12 lysine residues glucosylated per apolipoprotein B) or LDL glucosylated in the presence of Na(CN)BH3, i.e., conditions not expected to occur in diabetes, inhibit receptor-mediated internalisation and degradation. Moderately glucosylated LDL is also readily recognized by cultured rat hepatocytes and porcine endothelial cells. Human monocyte-derived macrophages accumulate cholesteryl ester when incubated with acetylated LDL for 12 days but no enhanced cholesteryl ester formation was found when native or glucosylated LDL (3.3 lysines glucosylated per apolipoprotein B) were used.

Nonenzymatic glycation of immunoglobulins leads to an impairment of immunoreactivity.

Incubation of purified human and rabbit immunoglobulin G with glucose leads to covalent incorporation of the sugar into the protein, depending on glucose concentration, incubation time and pH. Furthermore, the level of glycated immunoglobulin G from normal and diabetic subjects has been determined using the thiobarbituric acid reaction. The median for glycated immunoglobulin G, expressed as mmol 5-hydroxymethylfurfural per mol IgG, obtained from 20 normal and 29 diabetic subjects was 62 and 107, respectively. Glucose incubation of immunoglobulin G purified from rabbit anti-human-transferrin serum, from human anti-varicella/zoster virus serum and from human anti-lues-spirochete serum, respectively, leads to a marked decrease in biological activity, as determined in a micro complement fixation test. Inactivation of specific antibody was dependent on incubation time and glucose concentration employed. Loss in complement-fixing activity was observed at glycation levels well comparable to those found in diabetics.

Different effects of reductive and nonreductive glucosylation on LDL-catabolism.

The effects of various degrees of reductive and nonreductive glucosylation of low density lipoprotein on its catabolism by human fibroblasts have been examined. Moderate glucosylation of LDL does not alter its interaction with the high affinity receptor at concentrations of 5-2000 micrograms LDL-cholesterol/ml. Only heavy glucosylation of LDL (more than 12 lysine residues glucosylated per apo B), i.e. conditions not expected to occur in diabetes, slows receptor-mediated internalisation and degradation. In contrast, impairment of LDL-catabolism has been found at even low degrees of reductive glucosylation. The possible reasons for the different properties of reductively and nonreductively glucosylated LDL are discussed.

On the nature of the glucose tolerance factor from yeast.

The aim of this study was to purify the glucose tolerance factor (GFT) from yeast, and to gain further knowledge on its chemical structure. Activity of GTF was determined by its potency to stimulate 1-14C glucose oxidation of rat adipose tissue, in vitro, in the presence of maximally effective insulin concentrations. While described procedures Toepfer, Merth, Polansky, Roginski and Wolf (1977) resulted in some 3-fold purification, specific activity was no more increased by further fractionation on HPLC and/or TLC. Instead separation on TLC yielded several fractions with GTF activity, two of them enriched in aspartate and cystine/cystein, respectively, being the most important. L-aspartate, when added at comparable concentrations in vitro displayed similar GTF-activity as did adenosine, another component present in yeast extracts. It is concluded that GTF-activity of yeasts is not attributable to a single molecular species but rather to the combined action of amino acids and nucleosides especially aspartate, and adenosine.

[Iodine deficiency, congenital goiter and congenital hypothyroidism. Results of TSH screening in southern Bavaria]. / Jodmangel, kongenitale Struma und kongenitale Hypothyreose. Ergebnisse des TSH-Screening in Südbayern.

Increased TSH values (greater than or equal to 20 microU/ml) were found in 422 of 168,660 neonates born in Southern Bavaria. In 40 neonates the diagnosis of congenital hypothyroidism was confirmed (1: 4,200). The iodine excretion of healthy neonates and their mothers was on average significantly lower in the Munich than the Hamburg region. No relationship between iodine supply and congenital hypothyroidism was demonstrated. On the other hand, iodine excretion in children with neonatal goiter (17% of those with hyperthyrotropinaemia detected by screening) and their mothers was significantly lower than in control groups in the same regions. In addition, there was a significant correlation between iodine excretion of the mothers and neonates. The results confirm the need for iodine prophylaxis which should also take into account the higher needs during pregnancy. Apart from prophylactic aspects, these measures would also be in the interest of greater effectiveness and selectivity of TSH screening since it would reduce the number of cost-intensive follow-up examinations.

Decrease by glucagon in peroxide generation by isolated hepatocytes.

Exposure of isolated rat liver cells to glucagon or dibutyryl cyclic AMP leads to a prompt decrease in the rate of cellular peroxide generation as evidenced by (i) a reduced rate of [14C]formate oxidation and (ii) a lowered steady-state concentration of catalase Compound I.