Calcium transport by pea root membranes: I. Purification of membranes and characteristics of uptake

Calcium transport has been studied using purified endomembrane vesicles from dark-grown roots of Pisum sativum L. Membranes from a mixed microsomal (non-mitochondrial) fraction showed ATP-dependent calcium uptake which was released by the ionophore A 23187, had a pH optimum of 7.2 and required Mg2+ for uptake. Membranes were further purified using a rapid sucrosedensity-gradient technique yielding vesicles suitable for transport studies, and were identified using marker enzymes. Uptake by plasma membrane, tonoplast, endoplasmic reticulum and Golgi apparatus was indicated. Uptake by membranes of low density (predominantly tonoplast) had a pH optimum of 7.2—7.4 and nucleotide specificity ATP> guanosine 5′-triphosphate > inosine 5′-triphosphate > ADP >, while that by high-density membranes had a pH optimum of 7.5—7.9 and less specificity for ATP. The importance of regulating sucrose concentrations in calcium transport studies was demonstrated.