Kinetics of succinic dehydrogenase inhibition by methyl parathion and its recovery by oximes and L-cysteine in hepatopancreas of Lamellidens marginalis.

Effects in vitro of methyl parathion on some kinetic constants of succinic dehydrogenase (SDH) in hepatopancreas of freshwater mussel, L. marginalis were studied. Altered pH vs. specific activity curves for SDH demonstrated significant inhibition by methyl parathion in buffered acidic, neutral and alkaline ranges. At high pH ranges IC50 (12.5 microM) of methyl parathion did not cause 50% inhibition enzyme as it did at neutral and acidic pHs. Activation energies (delta E) were found to be increased suggesting decreased efficiency of enzyme in presence of methyl parathion. Non-competitive inhibition with respect to activation by succinate was indicated by decreased maximal velocity (V) without change in Michaelis Menten constant (Km). Pyridine-2-aldoxime (25 microM), pyridine-4-aldoxime (15 microM) and L-cysteine (40 microM) neutralized the inhibition of SDH by methyl parathion (12.5 microM). The kinetic data suggests that inhibition of SDH by methyl parathion was pH and temperature independent.

Effects of some metabolic intermediates on AMP-deaminase activity in denervation atrophy.

Effects in vitro of glutamate, aspartate, fumarate and malate on substrate activation kinetics of AMP-deaminase in denervated and contralateral gastrocnemius muscles of the frog, Rana hexadactyla were studied. Glutamate increase the enzyme activity of both muscles by increasing maximal velocity (V) and decreasing Michaelis-Menten constant (Km), whereas, aspartate did not show any significant effect. Fumarate and malate exerted mixed type of inhibition by decreasing V and increasing Km of enzymes in both muscle preparations. The inhibitor constant (Ki) values suggest that the denervated muscle enzyme was more sensitive to fumarate and malate as compared to the contralateral muscle enzyme.

Variation in the catalytic potential of AMP deaminase during muscular fatigue.

Substrate- and co-factor-dependent kinetics of AMP deaminase were studied in normal and fatigued gastrocnemius muscles of frog. Normal muscle enzyme showed greater enzyme co-factor affinity than enzyme-substrate affinity as evinced by low Kp values. Fatigue phenomenon was found to decrease the catalytic efficiency of the enzyme by lowering the enzyme-substrate affinity more than the enzyme-co-factor affinity and enhancing activation energy values. Present study elucidates the low level of operation of adenine nucleotide deamination involving AMP-deaminase reacting-system during prolonged contractile stress.