Abstract
Dihydrofolate reductase has been purified 2000-fold from guinea pig liver, and 450-fold from guinea pig small intestine. The chromatographic properties, pH optima, and substrate and cofactor requirements of the two enzymes have been compared; the susceptibility of the enzymes to a series of activators and inhibitors also has been determined. No differences were noted between dihydrofolate reductases from the two sources; it is concluded that the unusually high resistance of the guinea pig to folate antagonists cannot be attributed to differences in the properties of small intestine and liver dihydrofolate reductase, or to differences between guinea pig dihydrofolate reductase and the analogous enzyme from other mammalian species.