Evaluation of peroxisomal heme in yeast

Heme is supposed to be transported into peroxisomes to form peroxisomal catalase [EC 1.11.1.6] in harmony with proliferation of the organelle because of the absence of the heme synthetic pathway in peroxisomes. We tried to understand the transport mechanism of peroxisomal catalase through the peroxisomal membrane from the aspects of a cofactor, heme, by measuring cellular and subcellular heme contents with the pyridine hemochrome method, independent of measuring the catalase activity. n-Alkane-grown Candida tropicalis cells, in which peroxisomes develop profusely, contained a larger amount of heme than the glucose-grown cells, and the increase well matched that of the catalase activity. The results of subcellular fractionation of the n-alkane-grown cells showed that, in peroxisomes, the catalase subunit and heme existed in a molar ratio of 1:1, indicating that excessively transported catalase subunit proteins or heme could not be present in peroxisomes and they formed a tetramer having four molecules of heme. From this quantification of peroxisomal heme, it was strongly suggested that the amounts of the catalase subunit protein and heme transported into peroxisomes should be stoichiometrically regulated at the step of their synthesis or/and transport.