On the folic acid activity of human blood.

Knowledge of folic acid and related factors present in the human circulatory system seems important from a fundamental point of view, and particularly because of the use of folic acid antimetabolites in experimental cancer therapy. Available data are fragmentary and conflicting, as is shown in condensed form by Table I. In Table I experiments involving the use of Taka-Diastase have been disregarded, since these results were shown to be meaningless because conjugases of the blood liberate folic acid from conjugates present in the enzyme preparation, rather than vice versa (3, 4). Our own earlier results (5), which were 5 to 10’ times as high as those reported by others, were obtained without the use of anticoagulants 01 extraneous enzyme preparations, and without removal of blood proteins, bacterial growth response being measured by titration rather than turbidimetrically, because of the presence of suspended protein. Further investigation was directed toward clarification of the factors responsible for the divergent results, specifically, the possible r61e of the presence of anticoagulants, the r&e of heat treatment (sterilization), and the r61e of the blood proteins. Experiments showed that different anticoagulants yield different results even though, in the concentrations used, they do not directly influence the growth of the assay organisms Lactobacillus casei and Streptococcus lactis R. A summary of these findings is included in Table I. It also became clear from these data that in the presence of blood prot,eins assay response is much higher than after their removal by precipitation. Because folic acid-active material may conceivably be lost by being adsorbed on or combined with blood proteins, the possibility of separating the active low molecular factors by dialysis was considered. It was decided, for the first phase of the investigation, not to attempt differentiation between blood cells and plasma nor to apply exogenous conjugase sources but merely to establish the conditions under which whole blood