Heme occupancy of catalase in hemoglobin-free perfused rat liver and of isolated rat liver catalase

Maximal heme occupancy, the maximal proportion of total catalase heme present in the form of Compound I, is found to be 0.4 both in the enzyme isolated from rat liver and in the peroxisomal enzyme as present in the intact cells of perfused rat liver. This indicates that the ratio of second order rate constants for catalatic decomposition and for formation of Compound I, $\text{k}{}_{\mathrm{4\prime }}\text{k}{}_1$, is equal in vitro and in vivo.Catalase was isolated from rat liver, and the extinction coefficients for Compound I and for cyanide-catalase at 640 minus 660 nm were determined. The measurement of heme occupancy of catalase in hemoglobin-free perfused rat liver was made possible by wavelength scanning as well as by dual wavelength absorbance photometry. Thus, Compound I and cyanide-catalase were demonstrated in the red region and in the Soret band region.Meeting the particular needs of organ photometry, specific metabolic transitions were used to visualize specific transitions of absorbing pigments. Compound I is specifically demonstrated by its decomposition by the hydrogen donor, methanol. A measure for total catalase heme is provided by formation of cyanide-catalase. The cyanide concentrations required are well below appearance of possible interference by other cyanide-binding hemoproteins at 640–660 nm.