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Inhibition of mammalian squalene synthetase activity by zaragozic acid A is a result of competitive inhibition followed by mechanism-based irreversible inactivation.
J Biol Chem. 1995 Apr 21; 270(16):9083-96.JB

Abstract

Squalene synthetase (SQS, EC 2.5.1.21) catalyzes the first committed step in the formation of cholesterol and thus represents an ideal site for selectively inhibiting sterol formation. Previous studies have demonstrated that the fungal metabolite, zaragozic acid A (ZGA-A), inhibits SQS activity by mimicking the substrate farnesyl pyrophosphate, the reaction intermediate presqualene pyrophosphate, or both, through a process that confers increased apparent potency in the presence of reduced enzyme concentrations, an observation consistent with either tight binding reversible competitive inhibition or mechanism-based irreversible inactivation. The studies outlined in this report provide multiple lines of evidence indicating that ZGA-A acts as a mechanism-based irreversible inactivator of SQS. 1) Inhibition of SQS by ZGA-A is dependent on the [SQS] present in the incubation reaction, and this inhibition is time-dependent and follows pseudo-first order reaction kinetics, exhibiting kobs values that range between 2 x 10(-4)/s and 23 x 10(-4)/s for [ZGA-A] within the log-linear range of the inhibition curve, and a bimolecular rate constant of 2.3 x 10(5) M-1s-1.2) SQS activity is titratable by ZGA-A, such that for each [ZGA-A] evaluated, inactivation exhibits a threshold [SQS] whereby enzyme activity at lower [SQS] is totally inhibited. 3) Time-dependent inactivation exhibits saturation kinetics with a Km for the process of 2.5 nM, which is approximately equal to the IC50 for SQS inhibition under these conditions, suggesting that inactivation results from selective modification of a functional group of the enzyme active center rather than from a nonspecific bimolecular reaction mechanism and that most, if not all of the inhibition results from irreversible inactivation. 4) Saturable, time-dependent inactivation occurs with similar inactivation kinetics for both the microsomal and trypsin-solubilized forms of the enzyme, indicating that irreversible inactivation by ZGA-A is not a consequence of membrane modification but is a direct effect of the inhibitor on the enzyme. 5) Inactivation is biphasic, exhibiting a rapid ("burst") phase followed by a second, pseudo-first order phase, similar to that previously noted for irreversible inactivators in other enzyme systems, and occurs even in the presence of 5 mM concentrations of the nucleophylic scavenger dithiothreitol, suggesting that the reaction between ZGA-A and SQS occurs at or near the active center prior to diffusion of reactive species out of the catalytic cleft. 6) Inactivation can be prevented through competition with the substrate, farnesyl pyrophosphate, further identifying the active center as the site of modification.(

ABSTRACT

TRUNCATED AT 400 WORDS)

Authors+Show Affiliations

Department of Metabolic Diseases, Pfizer Central Research, Pfizer Inc., Groton, Connecticut 06340, USA.No affiliation info available

Pub Type(s)

Journal Article

Language

eng

PubMed ID

7721822

Citation

Lindsey, S, and H J. Harwood. "Inhibition of Mammalian Squalene Synthetase Activity By Zaragozic Acid a Is a Result of Competitive Inhibition Followed By Mechanism-based Irreversible Inactivation." The Journal of Biological Chemistry, vol. 270, no. 16, 1995, pp. 9083-96.
Lindsey S, Harwood HJ. Inhibition of mammalian squalene synthetase activity by zaragozic acid A is a result of competitive inhibition followed by mechanism-based irreversible inactivation. J Biol Chem. 1995;270(16):9083-96.
Lindsey, S., & Harwood, H. J. (1995). Inhibition of mammalian squalene synthetase activity by zaragozic acid A is a result of competitive inhibition followed by mechanism-based irreversible inactivation. The Journal of Biological Chemistry, 270(16), 9083-96.
Lindsey S, Harwood HJ. Inhibition of Mammalian Squalene Synthetase Activity By Zaragozic Acid a Is a Result of Competitive Inhibition Followed By Mechanism-based Irreversible Inactivation. J Biol Chem. 1995 Apr 21;270(16):9083-96. PubMed PMID: 7721822.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Inhibition of mammalian squalene synthetase activity by zaragozic acid A is a result of competitive inhibition followed by mechanism-based irreversible inactivation. AU - Lindsey,S, AU - Harwood,H J,Jr PY - 1995/4/21/pubmed PY - 2001/3/28/medline PY - 1995/4/21/entrez SP - 9083 EP - 96 JF - The Journal of biological chemistry JO - J Biol Chem VL - 270 IS - 16 N2 - Squalene synthetase (SQS, EC 2.5.1.21) catalyzes the first committed step in the formation of cholesterol and thus represents an ideal site for selectively inhibiting sterol formation. Previous studies have demonstrated that the fungal metabolite, zaragozic acid A (ZGA-A), inhibits SQS activity by mimicking the substrate farnesyl pyrophosphate, the reaction intermediate presqualene pyrophosphate, or both, through a process that confers increased apparent potency in the presence of reduced enzyme concentrations, an observation consistent with either tight binding reversible competitive inhibition or mechanism-based irreversible inactivation. The studies outlined in this report provide multiple lines of evidence indicating that ZGA-A acts as a mechanism-based irreversible inactivator of SQS. 1) Inhibition of SQS by ZGA-A is dependent on the [SQS] present in the incubation reaction, and this inhibition is time-dependent and follows pseudo-first order reaction kinetics, exhibiting kobs values that range between 2 x 10(-4)/s and 23 x 10(-4)/s for [ZGA-A] within the log-linear range of the inhibition curve, and a bimolecular rate constant of 2.3 x 10(5) M-1s-1.2) SQS activity is titratable by ZGA-A, such that for each [ZGA-A] evaluated, inactivation exhibits a threshold [SQS] whereby enzyme activity at lower [SQS] is totally inhibited. 3) Time-dependent inactivation exhibits saturation kinetics with a Km for the process of 2.5 nM, which is approximately equal to the IC50 for SQS inhibition under these conditions, suggesting that inactivation results from selective modification of a functional group of the enzyme active center rather than from a nonspecific bimolecular reaction mechanism and that most, if not all of the inhibition results from irreversible inactivation. 4) Saturable, time-dependent inactivation occurs with similar inactivation kinetics for both the microsomal and trypsin-solubilized forms of the enzyme, indicating that irreversible inactivation by ZGA-A is not a consequence of membrane modification but is a direct effect of the inhibitor on the enzyme. 5) Inactivation is biphasic, exhibiting a rapid ("burst") phase followed by a second, pseudo-first order phase, similar to that previously noted for irreversible inactivators in other enzyme systems, and occurs even in the presence of 5 mM concentrations of the nucleophylic scavenger dithiothreitol, suggesting that the reaction between ZGA-A and SQS occurs at or near the active center prior to diffusion of reactive species out of the catalytic cleft. 6) Inactivation can be prevented through competition with the substrate, farnesyl pyrophosphate, further identifying the active center as the site of modification.(ABSTRACT TRUNCATED AT 400 WORDS) SN - 0021-9258 UR - https://www.unboundmedicine.com/medline/citation/7721822/Inhibition_of_mammalian_squalene_synthetase_activity_by_zaragozic_acid_A_is_a_result_of_competitive_inhibition_followed_by_mechanism_based_irreversible_inactivation_ L2 - https://linkinghub.elsevier.com/retrieve/pii/S0021-9258(17)49106-X DB - PRIME DP - Unbound Medicine ER -