The effect of picrylsulphonic acid on In vitro conversion of cyanide to thiocyanate by 3-mercaptopyruvate sulphurtransferase and rhodanese.

This study indicates that 3-mercaptopyruvate sulphurtransferase (MPST; EC 2.8.1.2) activity may serve as a useful in vitro indicator for the analysis of cyanide detoxification to thiocyanate. The time course and capacity of MPST to detoxify cyanide was equal to or exceeded that of rhodanese. Picrylsulphonic acid strongly inhibited purified rhodanese, but in the presence of mercaptopyruvate, it could increase the formation of thiocyanate catalysed by MPST. Formation of thiocyanate by MPST followed a linear time course and had a linear relation to enzyme level. However, substrate dependence did not produce linear Lineweaver-Burk plots when either mercaptopyruvate or cyanide concentration was varied. The differential effect of picrylsulphonic acid on the activities of these two enzymes was confirmed by using a crude kidney extract as the source of both enzymes. Picrylsulphonic acid may provide a useful scientific tool to examine which sulphurtransferase is most responsible for the detoxification of cyanide.