The dynamics of local kinetic parameters of glutamate dehydrogenase in rat liver

Robust regulation of hepatic pericentral amination by glutamate dehydrogenase kinetics

α-Ketoglutarate substrate inhibition kinetics of hepatic glutamate dehydrogenase amination activity confers robust control over the ratio between glutamate and ammonium.

Determination of phosphate-activated glutaminase activity and its kinetics in mouse tissues using metabolic mapping (quantitative enzyme histochemistry)

Phosphate-activated glutaminase (PAG) converts glutamine to glutamate as part of the glutaminolysis pathway in mitochondria. Two genes, GLS1 and GLS2, which encode for kidney-type PAG and liver-type PAG, respectively, differ in their tissue-specific activities and kinetics. Tissue-specific PAG activity and its kinetics were determined by metabolic mapping using a tetrazolium salt and glutamate dehydrogenase as an auxiliary enzyme in the presence of various glutamine concentrations. In kidney and brain, PAG showed Michaelis-Menten kinetics with a K_m of 0.6 mM glutamine and a V_max of 1.1 µmol/mL/min when using 5 mM glutamine. PAG activity was high in the kidney cortex and inner medulla but low in the outer medulla, papillary tip, glomeruli and the lis of Henle. In brain tissue sections, PAG was active in the grey matter and inactive in myelin-rich regions. Liver PAG showed allosteric regulation with a K_m of 11.6 mM glutamine and a V_max of 0.5 µmol/mL/min when using 20 mM glutamine. The specificity of the method was shown after incubation of brain tissue sections with the PAG inhibitor 6-diazo-5-oxo-L-norleucine. PAG activity was decreased to 22% in the presence of 2 mM 6-diazo-5-oxo-L-norleucine. At low glutamine concentrations, PAG activity was higher in periportal regions, indicating a lower K_m for periportal PAG. When compared with liver, kidney and brain, other tissues showed 3-fold to 6-fold less PAG activity. In conclusion, PAG is mainly active in mouse kidney, brain and liver, and shows different kinetics depending on which type of PAG is expressed.

Determination of glutamate dehydrogenase activity and its kinetics in mouse tissues using metabolic mapping (quantitative enzyme histochemistry)

Glutamate dehydrogenase (GDH) catalyses the reversible conversion of glutamate into α-ketoglutarate with the concomitant reduction of NAD(P)(+) to NAD(P)H or vice versa. GDH activity is subject to complex allosteric regulation including substrate inhibition. To determine GDH kinetics in situ, we assessed the effects of various glutamate concentrations in combination with either the coenzyme NAD(+) or NADP(+) on GDH activity in mouse liver cryostat sections using metabolic mapping. NAD(+)-dependent GDH V(max) was 2.5-fold higher than NADP(+)-dependent V(max), whereas the K(m) was similar, 1.92 mM versus 1.66 mM, when NAD(+) or NADP(+) was used, respectively. With either coenzyme, V(max) was determined at 10 mM glutamate and substrate inhibition was observed at higher glutamate concentrations with a K(i) of 12.2 and 3.95 for NAD(+) and NADP(+) used as coenzyme, respectively. NAD(+)- and NADP(+)-dependent GDH activities were examined in various mouse tissues. GDH activity was highest in liver and much lower in other tissues. In all tissues, the highest activity was found when NAD(+) was used as a coenzyme. In conclusion, GDH activity in mice is highest in the liver with NAD(+) as a coenzyme and highest GDH activity was determined at a glutamate concentration of 10 mM.

Adaptational changes in kinetic parameters of G6PDH but not of PGDH during contamination-induced carcinogenesis in livers of North Sea flatfish

Kinetic parameters of glucose-6-phosphate dehydrogenase (G6PDH) and phosphogluconate dehydrogenase (PGDH) were determined in situ in livers of marine flatfish flounder that were caught in unpolluted areas in the open sea and in the highly polluted river Elbe (Germany). Analysis was performed quantitatively in liver sections using valid enzyme histochemical methods and image analysis. G6PDH but not PGDH was strongly affected by contaminant exposure and subsequent carcinogenesis. G6PDH showed a gradual decrease in Vmax and Km for glucose-6-phosphate in extralesional normal-looking liver tissue. Hepatocellular carcinomas also showed a low Km, whereas the Vmax was upregulated. These findings are interpreted as follows: prolonged challenges of the livers by pollutants inhibit or inactivate G6PDH and this is compensated for by reduction in Km. In carcinomas, G6PDH levels are upregulated but the low Km values are kept to increase the NADPH production capacity required in cancer cells showing that posttranslational regulation processes are important to control cellular metabolism under various environmental conditions.

Ala-Pro-cresyl violet, a synthetic fluorogenic substrate for the analysis of kinetic parameters of dipeptidyl peptidase IV (CD26) in individual living rat hepatocytes

A new type of fluorogenic substrates for proteases based on the leaving group cresyl violet has been synthesized. Cresyl violet is not fluorescent when amino acids or peptide groups are attached but becomes highly fluorescent after proteolytic liberation. Its fluorescence shows linearity with concentration and barely any fading. The properties of Ala-Pro-cresyl violet as substrate for dipeptidyl peptidase IV (DPPIV) (CD26) for localization and quantification of its activity in individual freshly isolated living rat hepatocytes were investigated using confocal microscopy, image analysis, and flow cytometry. DPPIV activity was localized exclusively in patches at plasma membranes likely being bile canalicular domains. Activity was analyzed quantitatively in individual cells by capturing series of images in time. Production of fluorescence was analyzed on the basis of the series of digital images and it appeared to be nonlinear with time. By calculation of the initial velocity at time zero, activity of DPPIV per individual hepatocyte was calculated. Cresyl violet-dependent fluorescence appeared in a similar way when cells were analyzed by flow cytometry. A dipeptide phosphonate inhibitor inhibited production of fluorescence competitively with a Ki of 7 microM. K(m) values in individual hepatocytes varied in the range of 6-22 microM depending on the individual rat from which the hepatocytes were obtained, whereas the Vmax varied in the range of 4-16 nU. K(m) and Vmax values per individual rat were inversely correlated indicating posttranslational regulation of the kinetic parameters of DPPIV. This relationship was lost when membrane fractions of the same hepatocyte suspensions were analyzed. It is concluded that cresyl violet-based protease substrates are the compounds of choice to localize and quantify protease activity in living cells and tissues.