Effect of Soybean Trypsin Inhibitor on Methionine and Cystine Utilization

Abstract

Feeding unheated soybean or heated soybean with a single oral dose of crystalline soybean trypsin inhibitor to rats increases the amount of expired C¹⁴O₂ derived from a tracer dose of DL-methionine-2-C¹⁴. This increased oxidation of labeled methionine is suppressed by feeding supplementary cystine. In growth and protein efficiency studies, cystine supplements unheated soybeans, but has no effect when heated soybeans are fed in an otherwise complete semipurified diet. Unheated soybeans were fed at a level in the diet that would provide the rat's normal requirement for methionine, assuming adequate cystine from the soybean protein (0.17% available methionine). Under these conditions dietary supplementation with either 0.3% L-cystine or 0.3% DL-methionine gave equal growth responses. It is concluded that in the rat, feeding unheated soybeans does not selectively impair the availability or tissue utilization of methionine, but there is a metabolic block in the utilization of cystine for protein synthesis. Furthermore, this block appears to be caused in some unknown manner by the trypsin inhibitors naturally occurring in unheated soybeans.