STUDIES ON THE IDENTIFICATION OF A FOLATE COMPOUND OF HUMAN SERUM*

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Abstract
1. Folic acid activity in human serum for Lactobacillus casei is not made available to Streptococcus faecalis or Leuconostoc citrovorum by the action of hog kidney or chicken pancreas conjugase. 2. A new intermediate of methionine biosynthesis, N5-methyl tetrahydrofolic acid, has marked folic acid activity for L. casei but relatively little for S. faecalis or L. citrovorum. The fact that this material is a monoglutamate indicates the existance of a previously unknown class of folic acid-active materials: monoglutamates microbiologically active for L. casei, but relatively inactive for S. faecalis or L. citrovorum. 3. The possibility that folic acid activity in human serum may be due largely to such a monoglutamate form is supported by the demonstration that a concentrate of serum folic acid activity and N5-methyl tetra-hydrofolate migrates identically in four different solvent systems.