922 THE INHIBITORY EFFECT OF TIN PROTOPORPHYRIN (SN-P) ON HEME DEGRADATION

Sn-P is a potent competitive inhibitor of heme oxygenase, the enzyme which converts heme to bile pigment and carbon monoxide (CO), and has been used to suppress hyperbilirubinemia in experimental animals and man. These experiments were performed to determine which heme pools are affected by Sn-P. Hepatic heme(s) were labeled with delta-aminolevulinic acid (ALA)-5-14C, which does not label hemoglobin heme in the rat. The “free” heme pool was tested by injecting heme prepared in vivo from glycine-2-14C Adult (300g) rats were injected either with Sn-P (50uM/kg sq) or saline one hour before injection with either 10uCi ALA-5-14C or 0.9mg hemin (specific activity 3×104 DPM/mg in 1ml alkalinized rat plasma). Expired 14CO was then collected over the next 24 hours. Results were: These experiments indicate that a single injection of Sn-P significantly inhibits the degradation of hepatic (non-erythropoietic) hemes for 12 hours and exogenous heme for an even longer period. They further suggest that Sn-P may exert different degrees of inhibition of bilirubin (and CO) formation depending upon the heme pool(s) involved.