Molecular Pharmacology, Vol 4, 162-167, Copyright © 1968 by the American Society for Pharmacology and Experimental Therapeutics

The Essential SH-Groups in Folate Reductase

¹ Department of Experimental Therapeutics, Roswell Park Memorial Institute, Buffalo, New York 14203

The number of essential SH-groups per amethopterin binding site has been determined in several preparations of folate reductase derived from a subline of S-180 cells grown in vitro. The content of folate reductase in terms of amethopterin binding sites varied in the different preparations between 15 and 20 mµmoles per milligram of protein and was based on titrations with amethopterin. In every preparation two SH-groups per binding site were essential for enzymatic activity and amethopterin binding. These two were protected against 5,5'-dithiobis-(2-nitrobenzoic acid) by TPNH, folate, and amethopterin and by low temperatures. The remainder of the SH-groups, which varied in different preparations, could not be protected and reacted with SH-reagent without interfering with the enymatic activity or amethopterin binding capacity. Folate reductase was found to be sensitive to inactivation by urea, which also accelerated the reaction with the SH-reagent.

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ACKNOWLEDGMENTS The author wishes to express her appreciation to Mrs. L. Kenny and Miss E.-M. Suolinna for assistance in this study. This investigation was supported in part by research grant CA-04175 from the National Cancer Institute of the U.S. Public Health Service.