Degradation of protein in the rumen of the sheep. II. The action of rumen micro-organisms on amino acids.

Work has been in progress here for some years on the use made by the sheep of different kinds of protein under different conditions (to be published; for preliminary communications on certain aspects see Cuthbertson & Chalmers, 1950; Chalmers & Synge, 1950). The results obtained have laid stress on the degradation of protein nitrogen to ammonia in the rumen. The possible importance of this was minimized in a review article by Schieblich (1929) and seems to have been first explicitly recognized by Pearson & Smith (1943) on the basis of in vitro incubation ofprotein with rumen contents. Detailed physiological data, however, were first obtained by McDonald (1948a, b) who measured the concentration of ammonia in the sheep's rumen at different times after feeding various proteins, and also showed that appreciable quantities of this ammonia may be absorbed into the blood draining the rumen; the ammonia would then presumably to a large extent be converted to urea in the liver and so be excreted. McNaught & Smith (1947) and Elsden & Phillipson (1948) have reviewed nitrogen metabolism in the rumen. The studies made at this Institute have in fact shown that nitrogen retention and growth in sheep on various diets can be correlated inversely with the extent of ammonia production in the rumen. In parallel with this work it seemed desirable to try to find out what other products of protein breakdown are formed besides ammonia. The present paper describes some relevant in vivo phenomena observed by chemical analysis of samples of rumen liquor obtained from sheep in the course ofexperiments on protein utilization by Dr Margaret Chalmers and Dr R. L. M. Synge. The subsequent paper describes in vitro experiments with the mixed micro-organisms of such rumen liquors, which throw light on some of the chemical reactions undergone by protein during its degradation in the rumen. This work formed the subject of a Thesis submitted to Aberdeen University for the Ph.D. degree in 1951 and some of it was included in a preliminary communication (el-Shazly & Synge, 1950). It seemed probable, from information available * Present address: Faculty of Agriculture, Farouk University, Alexandria. on bacterial deamination of amino-acids (cf. el-Shazly, 1952), that volatile fatty acids (VFA's) might be formed at the same time as ammonia. The higher members of the normal series present in the rumen (C4 and higher) might be expected to arise to a significant extent from amino-acids as well as from carbohydrates; members of the branchedchain series, arising from the amino-acids valine, leucine and isoleucine, which usually contribute about 20% of the nitrogen of proteins, would not be expected to arise at all from carbohydrates. When this work was begun in 1948, none of the micromethods available for analysing VFA's gave a good separation of branchedand straight-chain isomers. Some ofthe C4 and (C4 and higher) fractions isolated by the procedure of Elsden (1946) were therefore subsequently examined by the oxidation method of Klinc (1934, 1938). The results so obtained, though not inconsistent with the more recent findings, are not given here since the gas-liquid partition chromatogram of James & Martin (1951, 1952) became available later; this permitted unequivocal resolution of branchedand straight-chain isomers and of C5 from C6 and higher homologues, as well as being both rapid and convenient in use.