Hepatic heme catabolism in cultured hepatocytes

Uncertainty persists concerning the role and importance of heme oxygenase in the catabolism of heme by hepatocytes. The products of heme oxygenase catalyzed heme catabolism are equimolar amounts of biliverdin IX..cap alpha.., CO, and iron. Previous reports from studies with rodent hepatocyte cultures have suggested the possibility that non-heme oxygenase pathway(s) predominate in the breakdown of hepatic hemoprotein heme. The authors have studied this question in cultured chick embryo hepatocytes, which retain normal regulation of heme metabolism and levels of cytochromes P-450 as in intact animals. Exogenous heme added to the culture medium with control chick embryo hepatocyte cultures was quantitatively converted to biliverdin IX..cap alpha... To study endogenous heme breakdown, cellular heme was labelled by exposing cultured cells to (5-/sup 14/C) 5-aminolevulinic acid (ALA). The hepatocytes were also treated with mephenytoin that increases cytochrome P-450, total hepatic heme and heme oxygenase. At various times after labelling heme, biliverdin, and CO were isolated and counted. For at least 8 hrs, the increase in CO radioactivity corresponded to the loss of radioactivity in heme. Beyond 1 h biliverdin was unstable in culture medium, but for 1 h after labelling (dpm BVIX..cap alpha.. + dpm CO) ..delta..dpm heme. All BV detected wasmore » the ..cap alpha.. isomer. They conclude that heme oxygenase accounts for both endogenous and exogenous heme breakdown by hepatocytes.« less