La glycéraldehyde‐3‐phosphate déshydrogénase du latex d'Hevea brasiliensis

The cytoplasmic serum of Hevea brasiliensis latex contains two glyceraldehyde‐3‐phosphate dehydrogenases. These enzymes have been isolated and purified. One of them has an absolute specificity for NAD (EC 1.2.1.12); the other utilizes mainly NADP and reacts only slowly with NAD; it is similar to the non‐phosphorylating enzyme discovered by Arnon (EC 1.2.1.9). The NAD‐dependent enzyme has a molecular weight which is analogous to that of the other species of glyceraldehyde‐3‐phosphate dehydrogenases. Its optimum is around 8 and its Km‐values for the substrate, NAD and Pi respectively are: 0.1 mM, 0.12 mM and 2.6 mM. The NADP‐dependent enzyme is probably composed of 4 subunits and its molecular weight is estimated to be about 200000. Its optimum pH is around 8.6 and it catalyses a reaction that is irreversible. Its Km‐values for the substrate, NADP and NAD respectively are: 1.2 mM, 10 μM and 12 mM. Influence of some effectors like NADH, NADPH and adenosine phosphates on the enzymatic reaction has been studied. Only NADH has an influence. It inhibits competitively the reaction and the Ki value of the order 13 μM. It appears on the other hand, taking into account the influence of cysteine, that the NADP‐dependent enzyme is more resistant to xodative agents than the NAD‐dependent enzyme. NADP‐dependent enzyme is also inhibited by PO42‐, Mg2+, Ca2+, 3‐phosphoglycerate, glyceraldehyde iodoacetamide and citrate.