Thiosulfate reductase (thiosulfate-thiol sulfurtransferase, (EC 2.8.1.
3) TSR) and rhodanese (thiosulfate-cyanide sulfurtransferase, (EC 2.8.
1.1) RDN) activities have been identified in crude extracts from the e
ukaryotic green alga Chlamydomonas reinhardtii. These activities co-pu
rified after several purification steps and no isoforms have been sepa
rated. Kinetic studies indicate that dithiols (DTE) exert a competitiv
e inhibition on rhodanese activity, the affinity for DTE being 28-fold
higher than that for cyanide. These results support the idea that bot
h activities are mediated by the same protein. The activities are not
sensitive to thiol reagents, whereas they are inhibited by specific ar
ginine-modifying reagents. The physiological role of this enzyme could
be basically the assimilation of S-sulfane by the alga.