反刍动物瘤胃饲料蛋白的降解及氨的产生与抑制

RUMINAL PROTEIN DEGRADATION AND AMMONIA PRODUCTION

在瘤胃蛋白降解的过程中,微生物蛋白水解酶的活力及蛋白质的可溶性在许多情况下是一个限速步骤。在瘤胃液中游离氨基酸浓度很低时,小肽物质的累积给瘤胃微生物的摄取与利用提供了有效的基质,表明小肽(而不是游离氨基酸)是瘤胃微生物直接摄取的主要氮源,其摄取速度也是一个限速步骤。瘤胃中的 Bacteroides ruminicola Megasphaera elsdenii 和 Selenomonas ruminantium 曾被认为是主要产氨菌,但对瘤胃氨代谢中的许多矛盾现象仍难于解释。而新分离的瘤胃高效产氨菌(Strains C,F and SR)的产氨速度大约是他们的20—30倍。这些高效产氨菌只能生长在小肽或氨基酸中,并对瘤胃素(5μM)十分敏感;虽然这些产氨菌不能有效地降解蛋白质,但它们可以利用其他瘤胃细菌的蛋白酶或肽酶水解的产物小肽与氨基酸。这些高效产氨菌的总数不超过瘤胃菌群的5%,但产氨量却占30—50%。从而使一些瘤胃氨代谢中矛盾的现象得到合理的解释,并为今后对瘤胃产氨控制的研究建立了一定的基础。

Protease activity of microorganisms and the solubility of proteins are often limiting steps in ruminal protein degrada -tion.However,low concentrations of free amino acids,accumulation of small peptides,and rapid uptake and utilization of peptides by ruminal microorganisms indicate that small peptides (not amino acids) are major substrates for micrcorgan- isms and the rate of peptide uptake could also be rate—limiting.Bacteroides ruminicola,Megasphaera elsdenn,and Se- lenomonas ruminantium were previously considered to be the most important ammonia producers in the rumen,but many contradictions in ruminal ammonia metabolism could not be explained by these organisms.Recently isolated high ammonia —producing bacteria (strains C,F,and SR) were able to produce 20—30 times more ammonia than previous isolates. These strains were only able to grow on peptides or amino acids and were very sensitive to monensin (5μM).Although these ammonia producers have limited ability to degrade proteins,they can utilize peptides or amino acids released by the proteases and peptidases of other ruminal bacteria.Even though the total number of these ammonia producers is less than 5% of the microbial population,they may account for 30—50% of ammonia production in the rumen.The isolation of these high ammonia—producing bacteria explains many of the previous contradictions in the ruminal ammonia metabolism and provides a background for further study of ammonia control in the rumen.