Studies on gastric proteolysis. 1. The proteolytic activity of human gastric juice and pig and calf gastric mucosal extracts below pH 5

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Abstract
The gastric juice of 25 normal subjects digested one or more of the substrates plasma protein, casein, human serum, albumin, gelatin, bovine plasma albumin and edestin with two pH maxima below 5. The two maxima range from pH 1.6 to 2.4 and from pH 3.3 to 4.0 for different individuals and with different substrates. The pH maxima of gastric juices from different subjects and of samples collected at different times from the same individual sometimes differed more widely, when tested on the same sample of substrate, than could be accounted for by the range of reproductibility of the pH maxima as determined with the same sample of gastric juice. The two optima are unaffected, within experimental error, by time of digestion from 1/2 to 6 hours or by changes of substrate concentration up to 8-fold. The gastric juice of 2 normal subjects digested blood plasma with three pH maxima, the 3d occurring at pH 2.8 and 2.9. In contrast with the action on most substrates, the gastric juice of 5 normal subjects digested egg albumin with only one maximum at pH 1.5-1.8. The gastric juice of the 2 subjects who digested plasma with three maxima digested egg albumin with two, the 2d maximum occurring at pH 2.5 and 2.8. The gastric juice of 2 normal subjects hydrolyzed the specific pepsin substrate, benzyloxycarbonyl-L-glutamyl-L-tyrosine, with one optimum at pH 4.0. Swine and calf gastric mucosal extracts possessed proteolytic properties similar to those of human gastric juice, except that calf extracts digested egg albumin with two maxima. The proteolytic activity of calf mucosal extracts at pH 3.5 is not caused wholly by rennin, which has a pH maximum in this region, but also by another enzyme which is present in extracts from which rennin has been removed. The two proteolytic pH maxima are shown to occur independently of the number of substrate components and of the buffer anions present in the digestion mixture. Gastric proteolysis in vivo might occur readily at pH 3.5-4.5. It is not necessary that an intragastric pH as low as 2.0 or 1.5 be achieved.