Isolation and characteristics of scallop sarcoplasmic reticulum with calcium transport activity

Sarcoplasmic reticulum with calcium transport activity has been isolated from the cross-striated adductor muscle of the scallop, which lives in cold (≤20°C) sea water, by using pH 7.0 buffer solution both to homogenize the tissue and to sediment the membrane fraction. The yield of the preparation was 60-100 mg protein from 100 g of the scallop muscle. Ca²⁺-activated ATPase protein of about 100 kDa accounted for 40-50precnt; of the protein preparation. The maximum activities of ATP-dependent, oxalate-facilitated calcium accumulation and Ca²⁺-ATPase were observed at a pH of about 7.0 and temperature of 20- 30°C, and their values were about 2 μmol Ca²⁺/mg of protein/min and about 3 itmol ATP hydrolysis/mg of protein/min, respectively. At 0°C, 10-20% of these activities was maintained, while at 37°C, the activities were irreversibly lost. The Ca²⁺-ATPase activity was half-maximally activated at about 0.3μM [Ca²⁺]. The ATPase activity exhibited non-Michaelian behavior with respect to ATP, with two different K_m values of ∼10 μM and 0.1-0.3 mM. GTP, CTP, and ITP were also hydrolyzed by the preparation at a rate of 10-30% of that of ATP. The preparation was stored at - 80°C with retention of function for about a year.