Sn-protoporphyrin (SnPP) suppresses generation of 14CO from hepatic heme labeled with delta-aminolevulinic acid (5-[14C]ALA) or from infused [14C]hemin in rats. SnPP administered 1 h before administration of 5-[14C]ALA virtually abolished the peak output of 14CO occurring 2-3 h after injection of this heme precursor, and during the succeeding 12 h reduced 14CO excretion by approximately 61% compared with controls. When [14C]hemin was infused, SnPP diminished 14CO excretion by approximately 50%. These findings, derived from experiments using radiolabeled endogenous and exogenous heme sources, establish conclusively that the synthetic metalloporphyrin SnPP inhibits the oxidative degradation of heme in the intact animal.

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