Human embryonic cytochrome p450s: Phenoxazone ethers as probes for expression of functional isoforms during organogenesis

Human embryonic tissues were investigated during the period of organogenesis with a combination of substrate probes, selective inhibitors and immunoprobes in terms of their capacity to express functional P450 isoforms. A series of phenoxazone ethers utilized as substrate probes revealed that human embryonic hepatic, pulmonary, renal, adrenal and cardiac tissues each contained a complement of functional P450 isoforms when analyzed between days 50 and 60 of gestation. Preparations of each of these tissues contained isoforms capable of catalyzing O-demethylation, O-deethylation, O-depentylation and O-debenzylation of the respective phenoxazone ethers. Investigations with chemical inhibitors and inhibitory antibodies as well as comparisons with vector-expressed, human P450 isoforms suggested that isoforms of P450 subfamilies 1A, 2B, 2C or 3A were not major contributors to any of the observed reactions. The P450-dependent reactions studied exhibited several unexpected and unusual characteristics including a preference for NADH over NADPH as the initial electron donor. Results were consistent with the concept that conceptal-specific P450 isoforms participate in the human embryonic O-dealkylation/debenzylation probe reactions investigated.