EFFECT OF POLYAMINES ON GLUTAMATE-DEHYDROGENASE WITHIN PERMEABILIZED KIDNEY-CORTEX MITOCHONDRIA AND ISOLATED RENAL TUBULES OF RABBIT

The effect of polyamines on glutamate dehydrogenase {L-glutamate: NAD( P) oxidoreductase (deaminating) [EC 1.4.1.3]}activity has been studied in both permeabilized kidney-cortex mitochondria and isolated renal t ubules of rabbit. Spermidine was the most potent inhibitor of glutamat e synthesis in permeabilized mitochondria resulting in about 80% decre ase of the enzyme activity at 5 mM concentration. Putrescine, alpha-mo nofluoromethylputrescine (MFMP) and (R,R)-delta-methyl-alpha acetyleni c-putrescine (MAP) were more efficient than spermine. The inhibitory a ction of polyamines was potentiated by an elevated NADH content in the reaction mixture. Increasing concentrations of either NH4Cl, KCl or N aCl in the incubation medium resulted in a decrease of polyamine-induc ed inhibition of the enzyme activity, indicating that monovalent catio ns can compete with polyamines for the binding site at glutamate dehyd rogenase. The inhibitory action of spermidine on glutamate synthesis w as abolished by 2 mM ADP or 10 mM L-leucine, allosteric activators of the enzyme, as well as on the addition of either oxalate or sulphate a t 20 mM concentrations. Spermidine did not affect glutamate formation when NADH was substituted by NADPH, suggesting an importance of the NA DH binding to the inhibitory site of the enzyme for a decrease of redu ctive amination of 2-oxoglutarate by polyamine. Although spermidine di d not influence glutamate deamination in the presence of NAD(+), it st imulated this process by about 70% when NAD(+) was substituted by NADP (+). In the presence of ADP the stimulatory effect of polyamine was no t significant. The data indicate that in permeabilized rabbit kidney-c ortex mitochondria the effect of polyamines on both glutamate formatio n and glutamate deamination via the reaction catalysed by glutamate de hydrogenase is dependent upon the coenzyme utilized by the enzyme. In the presence of NADH their inhibitory effect on the glutamate formatio n may be alleviated by allosteric activators of the enzyme, and concen trations of potassium, sodium, sulphate and oxalate. In isolated rabbi t renal tubules incubated with 5 mM methionine sulfoximine and aminoox yacetate, in order to inhibit glutamine synthetase and aminotransferas es, respectively, 5 mM spermidine decreased glutamate formation by abo ut 30%, while putrescine and spermine did not significantly diminish t he enzyme activity. In the presence of octanoate glutamate formation w as reduced by about 30% by naturally occurring polyamines as well as M FMP and MAP, indicating that under these conditions NADH rather than N ADPH is utilized as the coenzyme. In view of these data it is possible to suggest that polyamines may be of importance to control glutamate dehydrogenase activity under physiological conditions.