title: Mechanistic interpretation of conventional Michaelis-Menten parameters in a transporter system.
abstract: The aim was to elucidate how steps in drug translocation by a solute carrier transporter impact Michaelis-Menten parameters Km, Ki, and Vmax. The first objective was to derive a model for carrier-mediated substrate translocation and perform sensitivity analysis with regard to the impact of individual microrate constants on Km, Ki, and Vmax. The second objective was to compare underpinning microrate constants between compounds translocated by the same transporter. Equations for Km, Ki, and Vmax were derived from a six-state model involving unidirectional transporter flipping and reconfiguration. This unidirectional model is applicable to co-transporter type solute carriers, like the apical sodium-dependent bile acid transporter (ASBT) and the proton-coupled peptide cotransporter (PEPT1). Sensitivity analysis identified the microrate constants that impacted Km, Ki, and Vmax. Compound comparison using the six-state model employed regression to identify microrate constant values that can explain observed Km and Vmax values. Results yielded some expected findings, as well as some unanticipated effects of microrate constants on Km, Ki, and Vmax. Km and Ki were found to be equal for inhibitors that are also substrates. Additionally, microrate constant values for certain steps in transporter functioning influenced Km and Vmax to be low or high.
authors.affiliation: University of Maryland School of Pharmacy, Baltimore, MD 21201, USA.
authors.forename: Diana
authors.initials: D
authors.lastname: Vivian
authors.affiliation: University of Maryland School of Pharmacy, Baltimore, MD 21201, USA. Electronic address: jpolli@rx.umaryland.edu.
authors.forename: James E
authors.initials: JE
authors.lastname: Polli
mesh_terms: D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism*; D004151:Dipeptides / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008562:Membrane Glycoproteins / Q000378:metabolism*; D017711:Nonlinear Dynamics; D011522:Protons; D027981:Symporters / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D004151:Dipeptides; D008562:Membrane Glycoproteins; D011522:Protons; D027981:Symporters; C037643:bile acid binding proteins
keywords: Apical sodium-dependent bile acid transporter; Michaelis–Menten; Model; Transporter