Effect of Penicillin Added to an Unheated Soybean Diet on Cystine Excretion in Feces of the Rat

Abstract

Fecal pellets were collected by removing them directly from the large intestine of anesthetized rats. The feces were lyophilized and analyzed for nitrogen, cystine, and trypsin activity. On the basis that trypsin contains 8.7% cystine (a value obtained by analyzing a commercial crystalline trypsin sample), the contribution of cystine in trypsin to the total fecal cystine was calculated. The rats were fed a diet containing unheated soybeans either with or without penicillin and the effect of preventing coprophagy was also measured. Penicillin in the diet increased the excretion of cystine in feces and this increase could be entirely accounted for by an increased excretion of trypsin. Under such conditions the rat must eat its feces in order to return the increased fecal cystine to the body so as to help provide for the large requirement for cystine in the synthesis of pancreatic protein secretions.