A TRANSALDOLASE - AN ENZYME IMPLICATED IN CRAB STEROIDOGENESIS

In arthropods, development is controlled by cholesterol-derived steroi d hormones: the ecdysteroids, In vertebrates and insects, steroidogene sis is positively regulated and this is mediated by cAMP. In crustacea ns, ecdysteroid biosynthesis by steroidogenic organs (Y-organs) is neg atively regulated by a neuropeptide, the Molt Inhibiting Hormone (MIH) . This neuropeptide-induced inhibition occurs via cyclic nucleotides a nd depends on protein synthesis. In the present work, we provide evide nce that a major 36.2-kDa cytosolic protein (P36; pi: 6.8) from crab Y -organs is positively correlated with steroidogenic activity. On the b asis of its amino acid sequence, P36 could be related to transaldolase , an enzyme of the pentose phosphate pathway which generates NADPH. In Y-organs, the enzymatic activity of Carcinus transaldolase increases with steroidogenic activity, and MIH treatment decreases both synthesi s and activity of transaldolase. Various transaldolases have been char acterized in very distantly related groups, namely bacteria, yeasts, a nd humans, These enzymes are highly conserved and present strong struc tural homologies, interestingly the crab transaldolase is closest to t hat enzyme characterized in human cells.