プリン塩基の分解と動物進化

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  • Purine Degradation and Animal Evolution
  • プリン エンキ ノ ブンカイ ト ドウブツ シンカ

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The end product of purine metabolism varies from species to species. In man and other primates and birds, the end product is uric acid because they do not possess uricase (EC 1.7.3.3) activity. In mammals other than primates and some reptiles, the end product is allantoin ; uricase is responsible for the hydrolysis of uric acid to allantoin. In most fishes and amphibia, the allantoin formed is further degraded to urea and glyoxylate via allantoic acid by allantoinase (EC 3.5.2.5) and allantoicase (EC 3.5.3.4). In some marine invertebrates and crustaceans, the urea formed is hydrolysed to NH_3 and CO_2 by urease (EC 3.5.1.5). Because the degradation of purines is much less complete in higher animals, it is apparent that certain enzymes have been lost during animal evolution, e.g. uricase, allantoinase, allantoicase, and urease. In the present study, the subcellular distribution and properties of enzymes involved in purine degradation in fish and crustaceous liver were examined. 1. In mackerel, yellow mackerel and prawn liver and mantis crab hepatopancreas, adenase, guanase and xanthine oxidase were present only in the cytosol, uricase and allantoinase both in the peroxisomes and in the soluble fraction, and allantoicase only in the peroxisomes. 2. Uricase and allantoinase were highly purified and characterized from the peroxisomal and soluble fractions from mackerel liver. Peroxisomal and soluble uricase and allantoinase showed nearly identical physical and enzymatic properties, respectively : uricase (allantoinase) had a molecular weight of approximately 128, 000 (48, 000), an isolectric point of 7.8 (5.3), a pH optimum at 8.6 (7.5) and an apparent K_m value of 13μM (11mM) for urate (allantoin). 3. The evidence was obtained that both soluble uricase and allantoinase activities were from broken peroxisomes. 4. Intraperoxisomal localization of uricase, allantoinase and allantoicase was examined in mackerel, yellow mackerel and prawn liver. Uricase and allantoinase were present in the soluble matrix and allantoicase in the membrane. These results suggest that in the metabolism of purines, peroxisomal enzymes have been lost during animal evolution : uricase, allantoinase and allantoicase.

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