Physiological role of 3-hydroxykynurenine and xanthurenic acid upon crustacean molting

Studies with crabs (Charybdis japonica) and crayfish (Procambarus clarkii) revealed that the tryptophan metabolites, 3-hydroxy-L-kynurenine (3-OH-K) and xanthurenic acid (XA), common secretory products of the X-organ-sinus gland complex of eyestalks from several decapods, regulated the molting of crustaceans in species-nonspecific fashion. Injection of 3-OH-K to the eyestalk-ablated crayfish delayed the onset of the first molt and lengthened the interval between the first and second molts. These lines of evidence were in accord with previous accounts of the so-called "molt inhibiting hormone" (MIH) effect. Removal of eyestalks caused a change in the conversion capacity of exogenous 3-OH-K to XA in the hemolymph. The peak in transformation capacity was followed by a peak in the titer of 20-hydroxyecdysone or molting hormone. Moreover, the seasonal profiles of the XA and ecdysone titers in Charybdis japonica exhibited a staggered relationship in the tissues tested. The ratio of XA to 3-OH-K, which is expected to indicate the apparent 3-OH-Kase activity, fluctuated seasonally and locally. When the Y-organ with the adhering tissues (Y-organ complex or YOC) was incubated during the period of high XA titer, the YOC produced 100 times more ecdysone than before incubation. It is suggested that ecdysteroidogenesis in situ was suppressed during this period by XA, but incubation of the YOC lead to a dramatic acceleration in ecdysone synthesis by overriding this inhibitory effect. XA profoundly repressed ecdysteroidogenesis in the YOC culture. Thus, XA is the ecdysone biosynthesis inhibitor (EBI) and 3-OH-K the precursor in crustaceans. An interfering effect of XA to a biocatalyst cytochrome P-450 system was postulated for the inhibition mechanism of ecdysteroidogenesis.