The effect of ethanol on the proteolytic activity of pepsin towards gastric mucus glycoprotein was investigated. Undegraded mucus glycoprotein isolated from pig stomach was incubated with pepsin in the presence and absence of ethanol, and the released α-amino residues were quantitated by the trinilrophenylation procedure. In the absence of ethanol the rate of proteolysis was proportional to mucus glycoprotein concentration up to 450 μg and remained constant with time of incubation for at least 30 min. The apparent Km value of pepsin for pig gastric mucus glycoprotein was 8.7 × 10-7 M. The proteolytic activity of pepsin towards mucus glycoprotein was inhibited by ethanol. The rate of inhibition was proportional to the ethanol concentration up to 0.8 M and was of competitive type. The apparent KI value calculated from the double-reciprocal plots for ethanol was 2.2 × 10-6 M. The inhibitory effect of ethanol on the proteolysis of gastric mucus glycoprotein may be of physiological significance.
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