INHIBITION OF BILE-ACID CONJUGATION BY CYCLOSPORINE-A

Each of the two steps involved in bile acid conjugation was tested in vitro for its sensitivity to inhibition by cyclosporin A (CsA). Bile a cid-CoA: glycine/taurine N-acyltransferase, the enzyme which catalyzes the second step, was tested and found to be insensitive to inhibition by 20 mu M CSA. Bile acid:CoA ligase, the enzyme which catalyzes the first step, was found to be inhibited by 25% at 10 mu M CsA in the sta ndard assay. The inhibition was competitive vs. bile acid and noncompe titive vs. ATP, and uncompetitive vs. CoA. CsA was also found to inter fere with the divalent cation requirement of the enzyme at low concent rations of Mg2+ the maximum inhibition was 70%. The maximum inhibition obtainable at physiologic Mg2+ concentration was 40%. The extent of i nhibition was presumably limited by the insolubility of CsA. At concen trations of CsA reached in vivo during drug therapy, CsA can be expect ed to significantly inhibit bile acid conjugation.