Investigations about influence of the content of plant crude protein in the ration on the utilization of urea in dairy cattle. 6. Digestive enzymes and their interactions in contents of proximal small intestine.

Forty-two samples were taken from the contents of the proximal small intestine of two lactating dairy cows fitted with re-entrant duodenal cannula. Most samples were free of detectable amylase activity. The (chymo)trypsinogen present was only partially activated to (chymo)trypsin. The activation was continued in vitro: slowly at the original pH of the samples (between pH 2.8 and 4.2), and faster after neutralization or a slight alkalinization. The effect of Ca, EDTA and soybean inhibitor on the activation of trypsinogen was also studied. The pancreatic enzymes were inactive in the acid pH range of the samples, but pepsin was markedly active. At pH 3.8 casein was digested rapidly by purified pepsin and slowly by the samples (agar-plate experiments). In model experiments performed with purified enzymes, pepsin digested (chymo)trypsin rapidly at pH 1-2 and slowly at pH 3.8. In the intestinal juice (chymo)trypsin and their zymogens seemed to be unaffected by pepsin under the conditions of the samples. It is concluded that the conditions prevailing in the duodenum/upper jejunum of the experimental cows account for a gastric-type digestion, despite the presence of pancreatic enzymes. In vivo the intestinal contents pass in distal direction. Meanwhile the pH of the chyme gradually increases and gives rise to first an increase of enterokinase activity accounting for a faster activation of the zymogens; second a start of function of activated pancreatic proteases and third a gradual decrease of pepsin activity and finally to its irreversible denaturation. Thus the development of intestinal type digestion is delayed in ruminants.