Heme (iron protoporphyrin IX) is the prosthetic moiety of cellular hemoproteins involved in vital cellular functions of all aerobic organisms. As such, its metabolism (synthesis and degradation) is very finely regulated to meet the prevailing cellular needs of the organism. Genetically determined enzymatic lesions in heme synthesis occur in humans, and are characteristic of the heme-deficient states, known as the porphyrias. In porphyric individuals, a variety of factors including drugs and diet frequently precipitate acute life-threatening attacks characterized by grave neuropsychiatric and abdominal symptoms. In these individuals, intravenous heme corrects the underlying biochemical abnormalities and dramatically relieves such symptoms. However, the mechanism of such heme-induced reversal remains to date obscure and enigmatic. Several mechanisms have been proposed, but substantive evidence in their support is lacking. In search of a common denominator, we examined other possible metabolic effects of acute heme deficiency. Our preliminary findings in porphyric rat models indicate that in acute heme deficiency, the activity of the hepatic hemoprotein, tryptophan pyrrolase markedly reduced. Such reduction results in substantially enhanced tryptophan uptake by the brain, and consequent stimulation of 5-hydroxytryptamine (5-HT) formation. This finding represents the very first identification of a specific CNS neurotransmitter pathway to be altered in hepatic porphyria. Proposed studies are designed to investigate whether, in fact, 5-HT is responsible for the neurological symptoms of acute hepatic porphyria. Hepatic heme is also critically required for the synthesis of cytochrome P-450, one of its most avid consumers. Since little is known about the structure-function relationships of this hepatic hemoprotein, studies are proposed to examine its structural assembly. Specifically, we will determine how critical heme is for the functional integrity of this cytochrome and whether in heme-deficient states other heme analogs could partially maintain cytochrome P-450 function. The orientation of the heme in the apocytochrome cleft will also be investigated. All of these proposed studies are expected to elucidate many heretofore unproved aspects of cytochrome P-450 structure and function.
