Kinetic properties and regulation of glycerol-3-phosphate dehydrogenase from the overwintering, freezing-tolerant gall fly larva, Eurosta solidagenis.

Abstract

The kinetic properties of cytoplasmic glycerol-3-P dehydrogenase from the third instar larva of the gall fly, Eurosta solidaginis, were studied with emphasis on temperature effects on the enzyme and the regulation of enzyme activity during the synthesis of the cryoprotectant, glycerol. Isoelectrofocusing revealed one major and two minor forms of the enzyme with no alteration in the pI's or relative activities of the forms in larvae acclimated to 24 versus −30 °C. Kinetic properties of the enzyme were also the same in larvae acclimated to high and low temperatures. Arrhenius plots were linear over a 30 to 0 °C range with an activation energy of 12,630 ± 185 cal/mol and a Q10 of 2.16. The Km for dihydroxyacetone-P was constant, at 50 μM, between 30 and 10 °C but increased by 75% at 0 °C; this increase may be a factor in the cessation of glycerol synthesis which occurs below 5 °C in this species. The Km(NADH), by contrast, was higher (5–6 μM) at 30 °C but decreased (3 μM) at lower temperatures. In the reverse direction, Km's were 340 μM for glycerol-3-P and 12 μM for NAD+. Effects of most inhibitors (of the forward reaction), glycerol-3-P (Ki = 2.4 mM), NAD+ (Ki = 0.2 mM), ATP, Mg·ATP, and Pi, were unaltered by assay temperature but ADP effects were potentiated by low temperature while citrate inhibition was greatest at high temperatures. Glycerol and sorbitol, which accumulate as cryoprotectants in E. solidaginis, had no significant effects on kinetic constants at any temperature but decreased the Vmax activity of the enzyme. Thermal inactivation studies showed an increased thermal stability of the larval enzyme compared to the homologous enzyme from rabbit muscle while added polyols stabilized enzyme activity, decreasing the rate of enzyme inactivation at 50 °C.