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An Agent Promoting Growth of Rats Fed Amino Acid Diets (Factor G)¹

Laboratory of Experimental Metabolic Diseases, Medical Research Programs, Veterans Administration Hospital, Long Beach, California, and Department of Biological Chemistry, School of Medicine, University of California, Los Angeles, California

An unidentified natural factor, designated factor G, is described. It enhances growth of rats fed amino acid diets containing all agents known to be required. Strong factor G activity was found in dried yeast, liver, kidney, and spleen, and also in egg albumin, lactalbumin, and crude casein. Inactive materials include dried heart, brain, and lymphatic tissue, and most proteins of plant and animal origin. The influence of factor G on growth is accompanied by a significant improvement in the efficiency of food utilization. The diet can be varied in many respects without interference with the factor G phenomenon. The factor G effect is not related to amino acid imbalance. Two basal diets, one imbalanced, the other of optimum amino acid composition, were used for detailed studies and regular factor G assays. Nearly identical results were obtained with both of them. Supplements of the amino acids were without factor G activity. Various substances known to function as growth factors in other systems were tested and found inactive.

Manuscript received 1 April 1970.