An improved assay for a growth factor in liver extracts.

Previouswork (1) has shown that liver contains a factor which produces a growth response in rats receiving a ration containing all the known vitamins. Bosshart et al. (2) have also described a growth-stimulating effect of liver in rats fed an otherwise complete diet. Jaffb and Elvehjem (3) and later Sporn et al. (4) described assay procedures for this factor. They observed a growth response of 5 to 10 gm. per week for periods of 3 to 5 weeks when liver preparations were given, but this response was not sufficient for a completely satisfactory assay. The present work was undertaken to improve the assay procedure and to study possible relationships between this factor and the antipernicious anemia factor found in liver. Ershoff (5) and Betheil et al. (6) have reported that liver completely counteracts the retardation of growth observed in immature rats when fed a diet high in thyroid. From a study of previous results (3, 4) it was observed that some of the rats on the basal ration grew as well as those on the supplemented ration. These data indicate that some of the rats had sufficient stores of the factor in question to grow normally to maturity, while those giving a response to liver suffered from a border line deficiency. It appeared possible that feeding thyroid or a thyroid-active substance as a supplement to the basal ration might increase the metabolic rate of the rat sufficiently to cause a greater requirement and thereby allow a more rapid depletion of the factor.