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A male inhibitory material (HuSePl-Frl) was separated from normal human seminal plasma by gel filtration on Sephadex G-100. This material has high avidity for cell membranes of human red blood cells (RBC). In vitro, PHA-M-induced blast transformation of human lymphocytes is inhibited by small quantities of HuSePl-Frl. This inhibition was also evident when RBC coated with HuSePl-Frl is added to mitogen stimulated lymphocytes in cultures. Small quantities of HuSePl-Frl are inhibitory to natural killer (NK) cell activation. The modulating role of the HuSePl-Frl in preventing the spermatozoal rejection during fertilization is discussed.

1. 1. An enzyme sysetm obtained from baker's yeast converted 2-methyl-4 amino-5-aminomethylpyrimidine to 2-methyl-4-amino-5-hydroxymethylpyrimidine, a known intermediate in thiamine biosynthesis. The pH optimum of the system was 7.8 and the apparent Km value for the substrate was 5·10−5 M. The enzyme system was inhibited by 2-mercaptoethanol, pyridoxal phosphate, the amine oxidase inhibitors pargyline and tranylcypromine, and by various carbonyl reagents and chelating agents. 2. 2. Enzymes systems catalyzing a similar reaction were reaction were found to be present in Neurospora crasa extracts and in a crude preparation of Bacillus thiaminolyticus thiaminase.