Quantitative cytochemistry of the diaminobenzidine cytochrome oxidase reaction product in mitochondria of cardiac muscle and pancreas.

The rate constant (k) of the cytochrome oxidase reaction under optimal conditions for cytochemical staining (i.e., 15 min fixation, incubation for 180 min for heart, 120 min for pancreas) can be used as a measure of the enzyme concentration within mitochondria. The rate constant derived from microdensitometric measurements of the mass thickness of the 3,3'-diaminobenzidine (DAB) cytochrome oxidase reaction in cristae times correlated data derived from morphometry on the surface density of cristae (SVcristae/Vmit micron-1) and the volume density of mitochondria per cell (Vmit/Vcell) has been used to determine the respiratory index (RI) of these tissues according to the following equation: RI = k(SVcristae/Vcell). Using this formula, the RI of cardiac muscle tissue was computed to be 33 times the RI of pancreas under the conditions of our experiments. The greater cristae surface density and the large mitochondrial volume density in cardiac muscle and high k value accounted for the higher RI of cardiac muscle.