The biochemistry of the metabolic fecal protein nitrogen.

In determining the nutritional value of various protein preparations in the infant in terms of nitrogen balance and growth response, it occurred to us that these results would gain in significance if the nature of the fecal proteins excreted under the various regimens were known. Although it is clear from the literature that the amount and composition of the fecal fat are to a large extent independent of the dietary fat (l), we were unable to find any clear evidence on the effect of dietary proteins on the composition of the fecal proteins (2). The attempt to assess the nature of this relationship raised the problem (a) of finding a suitable method for the quantitative determination of fecal proteins and (b) of isolating suitable quantities of these proteins for amino acid analysis. Heretofore only qualitative or indirect quantitative (3) procedures have been employed for the estimation of fecal proteins. After a number of preliminary experiments we found that the fecal protein could be’determined quantitatively by a micro-Kjeldahl analysis of the proteins precipitated isoelectrically at pH 6.0 from an alkaline ethanol extract of an aqueous homogenate of the feces. The analytical adequacy of this technique was further demonstrated by recovery tests and by its failure to measure proteins of the intestinal flora. Application of the method to the stools of ten infants on various diets for periods ranging from 1 to 16 weeks revealed that an average of 22.4 per cent of the fecal N arises from the protein moiety and that fluctuations from this mean value are spontaneous and not related to the diet. Analyses of fecal proteins isolated by a modification of the analytical procedure from the stools of these infants disclosed that the amino acid composition of these products was unaffected by the use of whole milk proteins, casein-, tryptophan-, isoleucine-, or lysine-deficient preparations as the nitrogen component of the diet. This observation, the absence of bacterial bodies from the protein fraction, and the uniformity of amino acid pattern of the different fecal protein specimens suggest that this stool fraction is not derived from residues of the dietary N or from the bodies of the intestinal bacteria, but from definite intestinal secretions.