The relationship between the structure and function of alkaline phosphatase isoenzymes is under investigation in a number of laboratories. The present study deals with the effects of glycosidase or protease digestion on the alkaline phosphatase isoenzymes. Changes in physicochemical properties, catalytic activity, affinity for various lectins and blood group antisera, carbohydrate composition and biological half-life were investigated.
1. Purified intestinal enzyme from adults was found to contain little sialic acid, whereas the fetal enzyme was found to be a sialoglycoprotein. Marked differences were observed with respect to the content of other sugars as well. Howeevr, the carbohydrate compositions of adult liver and fetal intestinal enzyme preparations were found to be similar.
2. Km Value of sialidase-treated hepatic enzyme was changed to a value near of the enzyme from intestine. But, pH dependence curves of the log Km value showed a marked difference between intestinal enzyme and hepatic one.
3. Desialylated hepatic enzyme was shown to be more heat labile and more sensitive to protease digestion in the presence of 0.5 % SDS than native hepatic enzyme.
Helix contents of native and desialylated hepatic enzymes were calculated to be 39.0% and 30.8/ respectively, apparent molecular weights 175,000 and 168,000, respectively.
4. Intestinal enzyme preparations treated with α-mannosidase, exo-N-acetyl-D-glucosaminidase and endo-N-acetyl-D-glucosaminidase-D displayed a decrease in enzyme activity. Among these, the α-monnosidase treated enzyme activity showed the most clearly reduction. By this treatment the maximum of the α-mannosidase-treated intestinal enzyme activity was shifted from 40mM Mg2+ to 5 - 10mM Mg2+.
5. Concanavalin A caused a biphasic modification of human alkaline phoshatase activity. The first stimulatory phase occurred at 0 - 0.02, μM, and the second inhibitory phase did at above 0.02, μM. The Hill coefficient was calculated to be about 0.5 in the stimulatory phase and 1.0 in the inhibitory one.
According to the electrophoretic studies of the interaction between intestinal enzyme and some lectins, its sugar chain would be similar to that of the circulating glycoprotein.
6. Papain-treated enzyme contained a hydrophobic and carbohydrate-rich peptide.
7. Purified alkaline phosphatases from adult human organ were associated with a sugar chain containing a blood group substance.
8. The half-life of human alkaline phosphatase was found to be 125 h from liver and 7.5 h from intestinal preparations, respectively. Desialylation reduced the half-life of human hepatic enzyme to 22-25 h, but any additional glycosidase digestion of the carbohydrate moiety restored the half-life to nearly that of the native enzyme.
Human intestinal enzyme with desialylation plus degalactosylation or with desialylation plus defucosylation showed a shortening of the half-life to about 1.2 h.
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