Enzymes of purine metabolism in the diatom,Phaeodactylum tricornutum

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Abstract
Cell-free extracts ofPhaeodactylum tricornutumcontained a methyltransferase which catalysed the methylation of guanine or hypoxanthine by 5–adenosyl-methionine. The product ofthe reaction appeared to be a methylhypoxanthine irrespective of whether guanine or hypoxanthine was substrate. This enzyme activity (per unit protein) trebled during 6 h of nitrogen deprivation. It is suggested that this enzyme is responsible for the methylation of guanine and hypoxanthine when these are taken up by nitrogen-deprived intact cells. Uricase, allantoinase and allantoicase were present in cells incubated with guanine, hypoxanthine, uric acid or allantoin;these activities did not appear during nitrogen-deprivation. Crude cell-free extracts containedaninhibitor of uricase and activity could be demonstrated only after ammonium sulphate precipitation of protein. When these three enzymes were induced by incubation with guanine, their order of appearance was identical to the order in which they operate in purine catabolism, namely (i) uricase (ii) allantoinase (iii) allantoicase. Guanine deaminase, xanthine oxidase and xanthine dehydrogenase activities were not detected irrespective of whether cells were nitrogen-deprivedor incubated with purine.
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