Casteels M, Schepers L, Van Eldere J, Eyssen H J, Mannaerts G P
Afdeling Farmakologie, Campus Gasthuisberg, Katholieke Universiteit Leuven, Belgium.
J Biol Chem. 1988 Apr 5;263(10):4654-61.
In isolated rat hepatocytes, fatty acids inhibited the side chain oxidation, but not the uptake, of exogenously added 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestan-26-oic acid (THCA). THCA did not inhibit fatty acid oxidation. In liver homogenates, fatty acids inhibited THCA activation to its CoA ester (THC-CoA) and THCA oxidation. THCA did not influence fatty acid activation or oxidation. Comparison of the THC-CoA concentrations present in the incubation mixtures during THCA oxidation, with substrate concentration curves determined for THC-CoA oxidation, indicated that the inhibition of THCA oxidation by fatty acids was at least partly exerted at the activation step. The inhibition of THCA activation by fatty acids was noncompetitive. Palmitoyl-CoA at concentrations found in the incubation mixtures during THCA oxidation in the presence of palmitate inhibited THC-CoA oxidation, but not sufficiently to fully explain the fatty acid-induced inhibition of THCA oxidation. The inhibition of THC-CoA oxidation by palmitoyl-CoA did not seem to be competitive. Acyl-CoA oxidase, the first enzyme of peroxisomal beta-oxidation (which catalyzes the side chain oxidation of THCA), was enhanced 15-fold in liver homogenates from clofibrate-treated rats when palmitoyl-CoA was the substrate, but the oxidase activity remained unaltered when THC-CoA was the substrate. In the perfused liver, oleate, infused after a wash-out period of 60 min, markedly inhibited bile acid secretion. The results 1) suggest that fatty acids inhibit THCA metabolism both at the activation step and at the peroxisomal beta-oxidation sequence and that separate enzymes may be involved in both the activation and peroxisomal beta-oxidation of fatty acids and THCA and 2) raise the question whether fatty acids might (indirectly?) affect overall bile acid synthesis via their inhibitory effect on THCA metabolism.
在分离的大鼠肝细胞中，脂肪酸抑制外源性添加的3α,7α,12α-三羟基-5β-胆甾烷-26-酸（THCA）的侧链氧化，但不抑制其摄取。THCA不抑制脂肪酸氧化。在肝脏匀浆中，脂肪酸抑制THCA活化为其辅酶A酯（THC-CoA）以及THCA氧化。THCA不影响脂肪酸活化或氧化。将THCA氧化过程中孵育混合物中存在的THC-CoA浓度与针对THC-CoA氧化测定的底物浓度曲线进行比较，表明脂肪酸对THCA氧化的抑制至少部分作用于活化步骤。脂肪酸对THCA活化的抑制是非竞争性的。在存在棕榈酸的情况下，THCA氧化过程中孵育混合物中发现的棕榈酰辅酶A浓度抑制THC-CoA氧化，但不足以充分解释脂肪酸诱导的THCA氧化抑制。棕榈酰辅酶A对THC-CoA氧化的抑制似乎不是竞争性的。过氧化物酶体β-氧化的第一种酶酰基辅酶A氧化酶（催化THCA的侧链氧化），当以棕榈酰辅酶A为底物时，在经氯贝丁酯处理的大鼠肝脏匀浆中增强了15倍，但当以THC-CoA为底物时，氧化酶活性保持不变。在灌注肝脏中，在60分钟的洗脱期后注入油酸，显著抑制胆汁酸分泌。结果表明：1）脂肪酸在活化步骤和过氧化物酶体β-氧化序列中均抑制THCA代谢，并且脂肪酸和THCA的活化及过氧化物酶体β-氧化可能涉及不同的酶；2）提出了脂肪酸是否可能（间接？）通过其对THCA代谢的抑制作用影响整体胆汁酸合成的问题。
