Charles A. Nichol

On the mechanism of action of aminopterin.

Summary Rat liver slices form from synthetic folic acid (PGA) a factor (CF) utilized for growth by Leuconostoc citrovorum; this conversion is inhibited by aminopterin. In rats given PGA, a similar inhibition of the synthesis of CF is shown by a marked decrease in the urinary excretion of CF following the administration of aminopterin. The antagonist not only prevents the metabolic alteration of PGA but also competes with the product (CF) derived from PGA. The lethal action in rats of aminopterin, 25 μg daily, is not prevented by PGA, but the daily administration of 250,000 units of CF completely counteracts the toxic effects of the antagonist. That CF is a biologically active derivative of PGA is shown by the fact that rats in which growth is arrested by PGA-deficiency, and which due to aminopterin are refractory to PGA, grow remarkably when given concentrates of the citrovorum factor.

Synthesis of Citrovorum Factor From Folic Acid By Liver Slices; Augmentation By Ascorbic Acid.

Summary Rat liver slices, normal or folic acid-deficient, are shown to form in the presence of pteroylglutamic acid a factor required for the growth of Leuconostoc citrovorum 8081. Ascorbic acid augments significantly the yield of the factor. The relationship of ascorbic acid to the metabolic activity of folic acid is discussed.

Resistance to folic acid analogues in a strain of Streptococcus faecalis.

Summary Bioautographs of several 4-amino analogues of folic acid indicated the presence of contaminants which corresponded to the demethylated, deaminated, or pteroic acid analogues. All the antagonists studied contained sufficient pteroylglutamic acid or pteroic acid to account for their apparent utilization for growth by an antagonist-resistant strain of Streptococcus faecalis. This strain had a lower requirement for PGA and pteroic acid and a much greater capacity to convert PGA to CF than did the parent antagonist-sensitive strain. Significant inhibition of the formation of CF from PGA by the resistant cells was obtained only with very high concentrations of Aminopterin or A-methopterin.

On the metabolic alteration of pteroylglutamic acid.

Summary The liver appears to be the main site of the conversion of pteroglyglutamic acid (PGA) to citrovorum factor (CF) in the body. Other tissues had very limited capacity to form CF, with the exception of bone marrow which, relative to the small amount present in this tissue, formed an appreciable quantity of the factor. The hepatic tissue of rats which had been depleted of folic acid and CF was flooded with CF within one hour following the intraperitoneal injection of PGA. Homogenates of rat or chick liver, when incubated with PGA under an atmosphere of nitrogen, were capable of forming CF. Little CF was found when similar preparations were incubated under oxygen. The yield of CF was consistently increased by the presence of ascorbate in the incubation mixture.