Comparison between biochemical and histochemical assessments of peroxidase activity in rat mammary tumours

The endogenous peroxidase content of 26 'hormone-dependent' and 16 'hormone-independent' rat mammary tumours was assessed by means of a biochemical (guaiacol) assay on tumour extracts and by means of a histochemical (diaminobenzidine) technique on frozen sections of the same tumours. By guaiacol assay, the hormone-dependent tumours had significantly higher peroxidase levels than the hormone-independent tumours. In contrast, by diaminobenzidine staining of the same tumours, peroxidase was detectable in 94% of hormone-independent tumours but in only 54% of hormone-dependent tumours. Moreover, there was no direct correlation between the results of biochemical and histochemical methods. At least in these rat mammary tumours, therefore, histochemical estimates of peroxidase activity based on the diaminobenzidine reaction do not seem to reflect the same tissue properties as biochemical estimates based on the guaiacol reaction after tissue disruption.