Chiral orientation of prosthetic heme in the cytochrome P-450 active site.

Sites of Covalent Attachment of CYP4 Enzymes to Heme: Evidence for Microheterogeneity of P450 Heme Orientation

“…The rate for 4-ipomeanol (IPO) bioactivation is expressed as (nmol/nmol P450)/30min. An alternative method for evaluating P450 heme stereochemistry has been developed that takes advantage of the alkylated heme adducts formed from 3,5-bis(carbethoxy)-2,6-dimethyl-4-ethyl-1,4-dihydropyridine (DDEP) during catalytic turnover (24). This method allows the analysis of heme orientation in mammalian P450s, for which no sufficiently high-resolution structures yet exist to discriminate between heme conformers.…”

Alkylation of the prosthetic heme in cytochrome P-450 during oxidative metabolism of the sedative-hypnotic ethchlorvynol

“…The proton assignments for the porphyrin framework are based on confirmed assignments of the same protons in other Nalkylated protoporphyrin IX derivatives. [19][20][21][22] The isolated porphyrin is unambiguously identified by these results as one isomer of dimethyl esterified N-(5-chloro-3-ethyl-3hydroxy-2-oxo-4-pentenyl)protoporphyrin IX (Figure 4).…”

“…Incubations were initiated by the addition of NADPH and were carried out at 37 °C. Heme concentrations were measured by the pyridine hemochromogen assay as reported previously.18 Hepatic green pigments were isolated by procedures analogous to those already reported.10,12, 19 In brief, ethchlorvynol (100 mg/kg dose) in polyethylene glycol 200 was administered ip to 250-300 g, Sprague-Dawley male rats that had been pretreated for 4 days with an 80 mg/kg daily intraperitoneal injection of sodium phenobarbital in water. The rats were decapitated 4 h after ethchlorvynol injection, and their livers were perfused in situ with cold isotonic saline before they were removed and homogenized in isotonic saline.…”

The biosynthesis of porphyrins, chlorophylls, and vitamin B12

“…The photochemistry of bilirubin has been well studied because phototherapy is the common treatment for the excessive levels of bilirubin (hyperbilirubinaemia) that cause neonatal jaundice. Both the 4-5 and the 15-16 double-bond can isomerize photochemically, to produce the less stable (E)-isomers (see Scheme 12), and the (40-isomer (55) photocyclizes more slowly to give cyclobilirubin (56) (which can then undergo further isomerization of the 15-16 double-bond). The cyclobilirubin isomers are the major photoproducts of bilirubin that are excreted by newborn infants during phototherapy.…”

“…In vivo, virtually all of the bilirubin is bound to serum albumin, and this alters the rates of the isomerization reactions dramatically. The rate constants have all been measured and it has been found that the isomerization of the (4Z)-compound to the (4E)-isomer is particularly slow when bilirubin is bound to human serum albumin.95 Thus while the (42,l 5E)-isomer can be readily detected by h.p.l.c., the (4E,lSZ)-isomer (55) does not accumulate to any appreciable extent because reversion to the (42,15Z)-isomer and cyclization [to form cyclobilirubin (56)] are both faster processes. In the photoisomerization of bilirubin that is bound to rat serum albumin, on the other hand, all of the steps are slow, and appreciable amounts of both (E, Z)-isomers can be detected.…”