When grown in the light and in a Tris-acetate phosphate medium, cells of Chlamydomonas reinhardtii Dang. can use the following L-amino acids as a sole nitrogen source: asparagine, glutamine, arginine, lysine, alanine, valine, leucine, isoleucine, serine, methionine, histidine, and phenylalanine, whereas, in the absence of acetate, the cells only used L-arginine. The utilization system in the acetate medium consisted of an extracellular deaminating activity induced by L-amino acids; it took between 10 to 30 h before the system appeared in cells previously grown with ammonium. This deaminase activity was nonspecific, required an organic carbon source for its de-novo synthesis, and was sensitive to high ammonium concentration and light deprivation.

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A Chlamydomona~' reinhardtff molybdenum cofactor (MoCo).carrier protein (CP), capable of reconstituting nitrate redugtase activity with apoprotein from the Neurospora crassa mutant nit.l, was subjected to experiments of diffusion through a dialysis membrane and gel filtration. CP bonded firmly MoCo and did not release it efficiently unles-~ aponitrate reduetase was present in the incubation mixture, Stability of MoCo bound to CP against air and heat was very similar to that of fre¢-MoCo released from milk xanthine oxidase. Our data strongly suggest that MoCo is directly transferred from CP to aponitrate reductase to form an active enzyme, Molybdenum ¢ofactor; Molybdopterin; Nitrate reductase; Xanthine oxidase

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Uricase (urate: oxygen oxidoreductase, EC 1.7.3.3) from leaves of chickpea (Cicer arietimum L.), broad bean (Vicia faba major L.), and wheat (Triticum aestivum L.) has been puri®ed to electrophoretic homogeneity by a procedure which includes xanthine-agarose anity chromatography as the main step. Puri®cation factors of 74 000±83 000 and recoveries of 80±90% were achieved. Puri®ed preparations had speci®c activities between 600 and 800 nkat á mg protein A1 (turnover numbers between 4400 and 6400 min A1 ). The three plant uricases were found by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to be tetramers of similar molecular mass (120±130 kDa) and to have identical or similar-sized subunits (32±34 kDa). They also had a similar optimum pH (9±9.5) and showed a hyperbolic kinetics with K m values from 9±24 lM. All of them showed similar responses to putative activators/ inhibitors. Oxonate, xanthine and, to a lesser extent, neocuproin inhibited uricase activity, whereas allantoin, ammonium, citrulline and glutamine did not. The three leaf uricases lacked catalase activity and were not activated by cadaverine. None of the three plant enzymes cross-reacted with anti-uricase monoclonal antibodies from soybean nodules or anti-uricase polyclonal antibodies from Chlamydomonas reinhardtii or rat liver. These results are consistent with the view that uricase in plants is probably a unique enzyme which is expressed at very low level in leaves.