Cited by 38Steroid Chemistry and BiochemistryMetal-Catalyzed Oxygenation MechanismsPharmacogenetics and Drug Metabolism

Side‐chain specificities of human and bovine cytochromes P‐450scc

TUCKEY, Robert C., CAMERON, Kathryn J.·1993·European Journal of Biochemistry

Abstract

Cytochrome P-450scc catalyses the conversion of cholesterol to pregnenolone by the sequential hydroxylation of the side chain of cholesterol. This occurs at a single active site and produces 22R-hydroxycholesterol and 22R-20 alpha-dihydroxycholesterol as intermediates. To further define the active site of human and bovine cytochromes P-450scc, we have examined the kinetics of the conversion of structural analogues of cholesterol with modified side chains, to pregnenolone. Analysis of the side-chain cleavage of analogues of cholesterol modified at C22 confirmed the high degree of structural specificity for the 22R position by cytochrome P-450scc, the major effect being on the turnover number (kcat) rather than on binding. The analogues of cholesterol that had a polar group at C24, C25 or C26 had much lower Km values and generally lower kcat values than the non-polar analogues which were tested. Km values of the polar analogues were 3-25-times lower than the Km for cholesterol and kcat values were also much lower than the kcat values for cholesterol, particularly for the human enzyme. The data suggest that the tight binding of the analogues with a hydroxyl or ketone group at C24, C25 or C26 places C20 and C22 in a poor orientation relative to the heme group for hydroxylation to occur. Many of the polar analogues which were tested are postulated regulators of cellular cholesterol metabolism. Several of these analogues are good substrates for bovine and human cytochromes P-450scc at low substrate concentration, as determined from their kcat/Km values. This study also indicates that the active site of cytochrome P-450scc is well conserved between bovine and human cytochromes. However, small species differences are evident since lower kcat values relative to the kcat of cholesterol are observed for some polar side-chain analogues of cholesterol with the human enzyme.

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Crossref · 10.1111/j.1432-1033.1993.tb18235.x

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  • Purified CYP11A1 also converts desmosterol and plant sterols including campesterol and β-sitosterol to pregnenolone.

    “The purified enzyme, also converts desmosterol and plant sterols including campesterol and β-sitosterol, to pregnenolone.”
  • Desmosterol is converted to pregnenolone by bovine, rat, and human CYP11A1 with similar catalytic efficiency to cholesterol.

    “Desmosterol (5,24-cholestadien-3β-ol, see Fig. 1), which has recently come to prominence, is also converted to pregnenolone by bovine, rat, and human CYP11A1 [16,17,19], with similar catalytic efficiency to that seen for cholesterol [48].”
  • Human and bovine CYP11A1 cleave the side chain of plant sterols campesterol and β-sitosterol producing pregnenolone, with catalytic efficiencies comparable to cholesterol for campesterol and 5-10 fold less for β-sitosterol.

    “Human and bovine CYP11A1 can also cleave the side chain of the plant sterols, campesterol (24α-methylcholesterol), and β-sitosterol (24β-ethylcholesterol) producing pregnenolone [19], with catalytic efficiencies comparable to cholesterol for campesterol and 5–10 fold less for β-sitosterol [48].”
  • Purified bovine and human CYP11A1 and steroidogenic mitochondria cleave the side chains of 24-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol.

    “These include cleaving the side chains of 24-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol (discussed above) [16,17,19,48].”

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Authors
TUCKEY, R. C., & CAMERON, K. J.
Affiliations
The University of Western Australia (AU)
Corresponding author
TUCKEY, Robert C.
Venue
European Journal of Biochemistry
Publisher
Wiley
Published
1993-10
Volume
217
Issue
1
Pages
209-215
ISSN
  • 0014-2956 (ISSN-L)
  • 1432-1033
OpenAlex
W1971605460
Cited by
38
References
31
Citations by year
2025: 1 · 2023: 2 · 2022: 1 · 2021: 1 · 2020: 1 · 2019: 1 · 2018: 1 · 2015: 1 · 2014: 2 · 2013: 1
FWCI
0.64
Type
journal article
Language
en
Open access
closed
Locations
  • European Journal of Biochemistry (publishedVersion)
  • PubMed (publishedVersion)
  • UWA Profiles and Research Repository (University of Western Australia) (submittedVersion)
Topics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Steroid Chemistry and Biochemistry
  • Physical Sciences › Chemistry › Inorganic Chemistry › Metal-Catalyzed Oxygenation Mechanisms
  • Life Sciences › Pharmacology, Toxicology and Pharmaceutics › Pharmacology › Pharmacogenetics and Drug Metabolism
Keywords
cytochrome P450scc, cholesterol side-chain cleavage, pregnenolone synthesis, cholesterol analogues, substrate specificity, active site, 22R-hydroxycholesterol, turnover number, Michaelis constant, species differences, cholesterol metabolism
MeSH terms
Adrenal Cortex, Animals, Binding Sites, Cattle, Cholesterol, Cholesterol Side-Chain Cleavage Enzyme, Female, Humans, Hydroxycholesterols, Hydroxylation, Ketocholesterols, Kinetics, Mitochondria, Placenta, Pregnenolone, Species Specificity, Substrate Specificity

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