Mode of action of specific inactivating enzymes for pyridoxal enzymes and NAD-dependent enzymes and their biological significance

Enzyme inactivation by a cellular neutral protease: enzyme specificity, effects of ligands on inactivation, and implications for the regulation of enzyme degradation

A protease from Tetrahymena pyriformis inactivated eight of nine commercially available enzymes tested, including lactate deyhdrogenase, isocitrate dehydrogenase (TPN-specific), glucose-6 phosphate dehydrogenase, D-amino acid oxidase, fumarase, pyruvate kinase, hexokinase, and citrate synthase. Urate oxidase was not inactivated. Inactivation occurred at neutral pH, was prevented by inhibitors of the protease, and followed first order kinetics. In those cases tested, inactivation was enhanced by mercaptoethanol. Most of the enzyme-inactivating activity was due to a protease of molecular weight 25,000 that eluted from DEAE-Sephadex at 0.3 M KCl. A second protease of this molecular weight, which was not retained by the gel, inactivated only isocitrate dehydrogenase and D-amino acid oxidase. These two proteases could also be distinguished by temperature and inhibitor sensitivity. Two other protease peaks obtained by DEAE-Sephadex chromatography had little or no no enzyme inactivating activity, while another attacked only D-amino acid oxidase. At least six of the enzymes could be protected from proteolytic inactivation by various ligands. Isocitrates dehydrogenase was protected by isocitrate, TPN, or TPNH, glucose-6-dehydrogenase by glucose-6-P or TPN, pyruvate kinase by phosphoenolypyruvate or ADP, hexokinase by glucose, and fumarase by a mixture of fumarate and malate. Lactate dehdrogenase was not protected by either of its substrates of coenzymes. Citrate synthase was probably protected by oxalacetate. Our data suggest that the protease or proteases discussed here may participate in the inactivation or degradation of a least some enzymes in Tetrahymena. Since the inactivation occurs at neutral pH, this process could be regulated by variations in the cellular levels of substrates, coenzymes, or allosteric regulators resulting form changes in growth conditions or growth state. Such a mechanism would permit the selective retention of enzymes of metabolically active pathways.

Regulation of breakdown and synthesis of L-glutamate decarboxylase in Clostridium perfringens

L-Glutamate decarboxylase (GAD) activity of Clostridium perfringens (ATCC 8009) cells grown in various culture conditions was investigated. Remarkable variations of GAD level occur during the growth cycle in thioglycollate broth. These changes are affected by the pH of the culture medium. Addition of alkali to the culture media results in decrease of cell GAD activity, whereas increase of enzyme level occurs only in cells growing in unbuffered media. The results indicate that the mechanism regulating the GAD levels is sensitive to the changes of pH (or buffering substances) rather than to the steady pH values. Neither repression by glucose nor induction by L-glutamate was observed. Moreover, high concentrations of the free amino acid substrate in the culture media considerably decrease cell GAD activity, owing to the buffering effect of the amino acid. The molecular mechanism supporting the variations of GAD activity during the growth cycle of the cells were investigated and tentatively related to the structural and functional properties of the pure enzyme. It is shown that the drop of GAD activity during the lag phase is due to protein breakdown. Evidence is presented suggesting a control of protein degradation by its quaternary structure. Data are also reported supporting de novo synthesis of GAD during the late logarithmic phase of cell growth. Finally, the possible role of GAD as part of the pH regulation system of C. perfringens cells is discussed in relation both to physiologic conditions of the bacterial cell and to the molecular mechanisms regulating the GAD activity in vivo.

Cold-induced increase of glycerol kinase activity in Neurospora crassa: rapid inactivation of the enzyme in vivo

The glycerol kinase activity induced by incubation of Neurospora crassa at low temperatures was rapidly lost when cultures were returned to 26 C. After a short lag, the activity disappeared irreversibly with a half-life of approximately 15 min. The loss of activity was not due to a change in the level of an inhibitor or activator. Glycerol reduced the activity loss but did not completely prevent it, which was an effect that was dependent on protein synthesis. The cold-induced activity was also always lost on addition of cycloheximide at all temperatures tested (0 to 26 C), which indicated continuous inactivation, although cycloheximide did not affect the actual rate of activity loss at 26 C. The basal glycerol kinase activity was not sensitive to cycloheximide. The mechanism responsible for inactivation was destroyed by sonic oscillation. The process is not thought to play a role in the cold-induced increase in activity. Glycerol kinase activity induced at 26 C by glycerol was also lost on addition of cycloheximide and after addition of sucrose.