Insect extramitochondrial glycerophosphate dehydrogenase II. Enzymic properties and amino acid composition of the enzyme from honeybee (Apis mellifera) thoraces

Several enzymic properties of crystalline honeybee (Apis mellifera) thoracic glycerophosphate dehydrogenase (L-glycerol-3-phosphate:DPN⁺ oxidoreductase, EC 1.1.1.8) were determined. The apparent Michaelis constant for dihydroxyacetone phosphate is 0.33 mM; there is no increase in activity with substrate concn. above 0.5 mM.

The bee enzyme has a broad pH optimum around pH 6.6, while the rabbits-muscle enzyme has a sharp optimum at pH 7.7. The bee enzyme is stable for 15 min at 21° from pH 4.8 to 9.9.

The temperature coefficient, $\text{Q}{}_{10}$, for the bee enzyme is 1.7 in the range from 21° 36°. The enzyme is stable for 5 min at 55° and completely inactivated at 61°. The bee enzyme shows the same relative reactivity with 2 DPN⁺ analogues as does the rabbit enzyme. The bee enzyme is inhibited by a low concentration of $\text{p-}\text{mercuribenzoate}$ (PCMB), is less sensitive to $\text{N-}\text{ethylmaleimide}$, and is not inhibited by 1 mM iodoacetate. Glutathione does not activate the enzyme.

The amino acid composition of the bee enzyme is quite different from the previously reported composition of the rabbits-muscle enzyme. The minimum molecular weight of the bee enzyme based on 1 tryptophan residue is 32 700. Since the molecular weight determined on a Sephadex G-200 column is around 67 000, the amino acid composition indicates a mol. wt. of 65 400.