Inhibition by Levorphanol and Related Drugs of Amino Acid Transport by Isolated Membrane Vesicles from Escherichia coli

Abstract

Levorphanol inhibits the transport of the amino acids proline and lysine by cytoplasmic membrane vesicles derived from Escherichia coli . The degree of inhibition increases with increasing levorphanol concentration and ranges from 26% at 10 −6 M levorphanol to 92% at 10 −3 M levorphanol. The effect is independent of the energy source, since levorphanol inhibits proline uptake to the same extent in the presence of 20 mM d -lactate or 20 mM succinate and in the absence of an exogenous energy source. Levorphanol does not irreversibly alter the ability of membrane vesicles to transport proline, since incubation of membrane vesicles for 15 min in the presence of 0.25 mM levorphanol, a concentration which inhibits proline transport by more than 75%, has no effect on the rate of proline transport by these vesicles once the drug is removed. Both the maximum velocity and the K m of proline transport are modified by levorphanol, hence, the type of inhibition produced by levorphanol is mixed. The inhibitor constant ( K i ) for levorphanol inhibition of proline transport is approximately 3 × 10 −4 M. Membrane vesicles incubated in the presence of levorphanol accumulate much less proline at the steady state than do control vesicles. Furthermore, the addition of levorphanol to membrane vesicles preloaded to the steady state with proline produces a marked net efflux of proline. Levorphanol does not block either temperature-induced efflux or exchange of external proline with [ 14 C]proline present in the intravesicular pool. Dextrorphan, the enantiomorph of levorphanol, and levallorphan, the N -allyl analogue of levorphanol, inhibit proline and lysine transport in a similar manner. Possible mechanisms of the effects of these drugs on cell membranes are discussed.