A transaldolase : An enzyme implicated in crab steroidogenesis.

In arthropods, development is controlled by cholesterol-derived steroid hormones: the ecdysteroids. In vertebrates and insects, steroidogenesis is positively regulated and this is mediated by cAMP. In crustaceans, ecdysteroid biosynthesis by steroidogenic organs (Y-organs) is negatively regulated by a neuropeptide, the Molt Inhibiting Hormone (MIH). This neuropeptide-induced inhibition occurs via cyclic nucleotides and depends on protein synthesis. In the present work, we provide evidence that a major 36.2-kDa cytosolic protein (P36; pl: 6.8) from crab Y-organs is positively correlated with steroidogenic activity. On the basis 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 ofCarcinus transaldolase increases with steroidogenic activity, and MIH treatment decreases both synthesis and activity of transaldolase. Various transaldolases have been characterized in very distantly related groups, namely bacteria, yeasts, and humans. These enzymes are highly conserved and present strong structural homologies, interestingly the crab transaldolase is closest to that enzyme characterized in human cells.