Dietary Factors in the Utilization of Homocystine

Abstract
Homocystine can replace methionine in the diet of Tetrahymena provided some factor or factors found in liver are present. The liver factor appears not to be a methyl donor as is indicated by the failure of methionine replacement by homocystine in the presence of choline. The liver factor can best be described as a cotransmethylase. A discussion of the mechanism or mechanisms of transmethylation in animals is given.