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Ca2+ binding and ATP dependent Ca2+ uptake by plasma membrane fraction were studied and the effect of treating plasma membrane fraction with trypsin, phospholipase c and neuraminidase on both activities was observed. Results of the above tests are as follows; 1. Plasma membrane fraction possessed the ability to bind Ca2+. The Scatchard plot of Ca2+ binding shwoed that plasma membrane had at least two types of Ca2+ binding sites of high and low affinity. 2. The amount of Ca2+ uptake by plasma membrane fraction was 18.51±1.31 nmoles Ca2+/mg protein/30min at 10-5M Ca2+. The Ca2+ concentration for half maximal activation in Ca2+ uptake was 3.7×10-7M. 3. The ATP dependent Ca2+ uptake by the microsome isolated from guinea pig intestine was clearly stimulated by 4mM K2-oxalate whereas that of plasma membrane fraction was not. 4. Trypsin and phospholipase c treatment led to a 40-88% reduction in the ATP dependent Ca2+ uptake. The Ca2+ concentration for half maximal activation Ca2+ uptake shifted to a high concentration. On the other hand, neuraminidase treatment resulted in a 43-93% increase in ATP dependent Ca2+ uptake. But, the Ca2+ concentration for half maximal activation in Ca2+ uptake was not shifted by neuraminidase treatment. The results show that plasma membrane plays an important role in regulating the cytoplasmic Ca2+ concentration. The results also suggest that a glycoprotein linked with sialic acid is involved in the flux of Ca2+ across the plasma membrane.
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