The relative potential of various structural isomers (III, XIII) and various 2,4-side chain modified analogs of heme (iron-protoporphyrin IX) to incorporate into rat liver hemoproteins, cytochrome P-450(s), and tryptophan pyrrolase was examined. Such assessments for hepatic cytochrome P-450 relied on generation of reconstitutible apocytochrome(s) P-450 by suicidal alkylation of the existing prosthetic heme moiety by allylisopropylacetamide (AIA) in vivo. Subsequent replacement of the prosthetic heme was brought about by incubating the apocytochrome(s) P-450-enriched preparations with a particular heme isomer or analog. Structure-function relationships of the reconstituted isozymes were assessed in microsomal preparations by monitoring cytochrome P-450 content (structure) and its mixed function oxidase activity (function). In parallel, the relative ability of these heme isomers and analogs to functionally constitute hepatic tryptophan pyrrolase was also assessed by monitoring the relative increase in holoenzyme activity when preparations deliberately enriched in constitutible apoenzyme were incubated with each of these compounds. The findings reveal that 2,4-side chain modifications on the heme IX skeleton markedly influence the function of the constituted hemoproteins possibly by affecting their structural assembly through steric, electronic, and/or hydrophobic interactions with the corresponding apoproteins. Furthermore, these studies not only reveal that the structural specifications of the active prosthetic site of rat liver cytochrome P-450(s) differ from those of tryptophan pyrrolase, but also that the structural specifications of these mammalian hemoproteins for their prosthetic heme differ considerably from those reported for their bacterial counterparts.