The Na+-induced efflux of Ca2+ catalysed by the Na+/Ca2+ carrier of cardiac mitochondria is strongly inhibited by extramitochondrial Ca2+. The nature of this inhibition was investigated as follows. (a) The apparent association of external Na+ and the Ca2+ analogue Sr2+ with substrate-binding sites (i.e. those sites involved in cation translocation) is promoted markedly by K+. The inhibition of Na+/Ca2+ exchange by external Ca2+ is affected little by K+. (b) There is a competitive relationship between the binding of external Na+ and external Ca2+ to substrate-binding sites, whereas at low concentrations (less than 4 microM) extramitochondrial Ca2+ is a partial non-competitive inhibitor with respect to external Na+. (c) This inhibiton by external Ca2+ is characterized by a maximal decrease of about 70% in the Vmax of Na+/Ca2+ exchange and by cooperative binding of external Ca2+ to sites that are half saturated by 0.7-0.8 microM free Ca2+. The binding of Ca2+ and Sr2+ to substrate-binding sites shows no co-operativity. These criteria suggest that the Na+/Ca2+ carrier may contain regulatory sites that render the carrier sensitive to changes in extramitochondrial [Ca2+] within the physiological range.
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Research Article| February 15 1982
Evidence for the existence of regulatory sites for Ca2+ on the Na+/Ca2+ carrier of cardiac mitochondria
Biochem J (1982) 202 (2): 509–518.
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L H Hayat, M Crompton; Evidence for the existence of regulatory sites for Ca2+ on the Na+/Ca2+ carrier of cardiac mitochondria. Biochem J 15 February 1982; 202 (2): 509–518. doi: https://doi.org/10.1042/bj2020509
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