Problems of nitrogen catabolism in invertebrates

Abstract

The sources and mode of formation of the excreted urea and uric acid in a terrestrial gastropod, H. pomatia, were studied by manometric technic, using tissue slices and metabolic exps. on the intact animal. The results show that only 1/40 part of the excreted urea can be derived from the urea of the normal diet. From in vitro exps. no evidence could be obtained of any mechanism synthesizing urea from the products of deamination. In general, the arginine of the diet appears to be responsible for the excreted urea. This is also the case in another terrestrial class, the birds. In vivo exps., however, show that additional urea excretion may follow the injection of amino-acids (especially with simultaneous carbohydrate), perhaps indicating the existence of a synthetic mechanism, or, with equal plausibility, a "diuretic" effect. The discovery of arginase and urease in Helix by previous workers was confirmed. It is of much evolutionary interest that a portion of the omithine cycle exists already in some invertebrates. The uricotelic character of this gastropod''s metabolism is supported by new in vitro exps. But the hepatopancreas of the snail contains no precursor which the pigeon-kidney can convert into uric acid, apart from an ample supply of substrates for xanthine oxidase. Conversely, the precursor substance known to be present in and to be formed by the pigeon-liver cannot be turned into uric acid by the snail-hepatopancreas. Thus the formation of uric acid from the ammonia of protein de-amination probably follows different courses in the 2 phyla. Xanthine oxidase is shown to be present in the hepatopancreas of the snail, though hitherto unrecorded from molluscs or crustaceans. This enzyme may be on the main path of formation of molluscan uric acid, whether derived ultimately from nuclein breakdown or from protein deamination. The catabolism of nucleoprotein alone cannot account for the quantities of uric acid present in the nephridium and from time to time excreted. If the molluscan hepatopancreas can be homologized with the liver, Clementi''s rule does not apply outside the vertebrate phylum, since in Helix arginase and uricotelic metabolism coexist. On the other hand, Przylecki''s rule, that uricotelic organisms never contain uricase, receives new support.