Transport and assimilation of purines in Chlamydomonas reinhardtii
M. Pineda and J. Cárdenas

Transport and assimilation of purines in the unicellular green alga Chlamydomonas reinhardtii are reviewed. Adenine, guanine, hypoxanthine, xanthine and urate are good nitrogen sources for growth of Chlamydomonas. These purines are transported by means of systems exhibiting saturable, hyperbolic kinetics lacking diffusion component, with very low Kt values and promoting the intracellular accumulation of purines against a concentration gradient. Transport systems are repressed by ammonium since purines were taken up by cells only after lag phases required to induce them and the ability to transport purines did not appear in the presence of protein synthesis inhibitors. As deduced from experiments of cross induction, cross-inhibition and the effect of many metabolic inhibitors, Chlamydomonas cells have two different systems for purine transport, one for adenine, guanine and hypoxanthine and another one for xanthine and urate. The properties of two key enzymes responsible for the oxidation of purines are described. Xanthine dehydrogenase (330 kDa) is a molybdoflavoprotein containing iron-sulfur centers, that catalyzes the oxidation of hypoxanthine and xanthine with NAD+ as the electron acceptor. The enzyme is rapidly and irreversibly inactivated by substrates in a process affecting mainly the molybdenum center. Urate oxidase is a cuproprotein of 124 kDa consisting of four similar-sized subunits that shows a strict specificity toward its substrates, urate and oxygen. This enzyme activity is competitively inhibited by xanthine, hypoxanthine, and oxonate. The intracellular accumulation of purines against concentration gradients, the differences in Michaelis constants and optimum pH values for transport and enzyme activities, as well as the different responses of uptake and enzymes too many metabolic inhibitors allow us to distinguish between purine transport and assimilation in Chlamydomonas cells.

Keywords: Purines, urate oxidase (uricase), xanthine dehydrogenase, Chlamydomonas.
Contents of this volume Sci. Mar. 60(Suppl.1) : 195-201 Back PDF
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