Phosphate transport protein of rat heart mitochondria: location of its SH-groups and exploration of their environment

(1) The properties of the SH groups of the phosphate transport protein of rat heart mitochondria were investigated on the basis of inhibition caused by SH reagents under different conditions. (2) The essential thiol groups are located near the external surface, as they are accessible to impermeable reagents from the external space. (3) The environment of the sulfhydryl groups influences their reactivity, as alteration of the external pH affects adversely their reactions with ionizable and non-ionizable SH reagents. (4) Intramitochondrial pH exerts a transmembrane effect: alkalinization augments and acidification diminishes the reaction rate of the sulfhydryl groups on the opposite surface of the membrane. (5) Changes of the concentration of the transported substrate occurring exclusively in the extramitochondrial space do not influence the reactivity of the essential SH groups. (6) It is concluded that in transport studies the phosphate transport protein of heart and liver mitochondria show basic similarity. It is suggested that the amino-acid sequence around the NEM-reactive cysteine (i.e., Lys-41 - Cys-42 - Arg-43) does not participate in substrate binding.