Uptake and metabolism of allantoin and allantoate by cells of Chlamydomonas reinhardtii (Chlorophyceae)

The green alga Chlamydomonas reinhardtii can use the ureides allantoin and allantoate as sole nitrogen sources. Once the uptake systems for allantoin and allantoate were induced, the uptake and growth rates were identical for the two ureides. However, the enzymatic activities involved in the degradation of the two ureides (allantoinase and allantoicase) were regulated differently. Allantoinase seems to be constitutive, since it was detected in all the nitrogen sources studied, while allantoicase behaved as an inducible enzyme, since it was present only in cells cultured in ureides or any metabolic precursor of these compounds. Neither allantoinase nor allantoicase activities were repressed by ammonium in the presence of ureides. Allantoicase activity was not induced under nitrogen starvation conditions, while it was induced in cells that had been cultured with allantoin or allantoate in the dark. Allantoin uptake showed a pattern similar to that of allantoate under all nutritional and environmental conditions tested. Inhibition of allantoin and allantoate uptake by N-ethylmaleimide suggests that thiol (SH-) groups are involved in both uptake systems. The use of both allantoin and allantoate was similarly inhibited by the metabolic poisons tested (cyanide, azide, 2,4-dinitrophenol and 3'-(3,4-dichlorophenyl)-1',1'-dimethyl urea) but only at very high concentrations. The possibility that uptake of allantoin and allantoate might take place through two independent systems is discussed.