Vessey D A, Kelley M
Liver Study Unit, Department of Veterans' Affairs Medical Center, San Francisco, CA 94121, USA.
Biochim Biophys Acta. 1998 Feb 17;1382(2):243-8. doi: 10.1016/s0167-4838(97)00163-5.
The XL-I, XL-II and XL-III forms of xenobiotic/medium-chain fatty acid: CoA ligase were found to be inactive toward benzoate in the absence of either monovalent or divalent cations. The absolute requirement for monovalent cation was satisfied by either K+, Rb+, or NH4+. Na+ only supported a very low rate. Varying the nature of the anion had only a minor effect. For XL-I and XI-II, the optimum concentration of K+ was 50 mM; higher (physiologic) concentrations led to a decrease in activity. K+ did not inhibit XL-III. The absolute requirement for divalent cation was satisfied by Mg2+ or Mn2+, or to a lesser extent by Co2+ or Fe2+. For the XL-I and XL-II, excess uncomplexed Mg2+ or Mn2+ decreased the rate; the optimum concentration of Mn2+ was approximately the same as the concentration of ATP in the assay, and the optimum concentration of Mg2+ was approximately double the concentration of ATP in the assay. This is consistent with the concept that the divalent cation is required to complex with ATP and with the known stability constants for the ATP complexes of these two divalent cations. XL-III was not inhibited by uncomplexed divalent cations. Uncomplexed ATP was a moderate inhibitor of XL-I and XL-II, and a weak inhibitor of XL-III. The data indicate that in vivo benzoate conjugation is K+ and Mg2+ dependent, and that the cation effects are complex and differ for XL-I and XL-II as compared with XL-III.
在缺乏一价或二价阳离子的情况下，发现外源性物质/中链脂肪酸：辅酶A连接酶的XL-I、XL-II和XL-III形式对苯甲酸酯无活性。一价阳离子的绝对需求可由K⁺、Rb⁺或NH₄⁺满足。Na⁺仅支持非常低的速率。改变阴离子的性质仅有轻微影响。对于XL-I和XL-II，K⁺的最佳浓度为50 mM；更高（生理）浓度会导致活性降低。K⁺不抑制XL-III。二价阳离子的绝对需求可由Mg²⁺或Mn²⁺满足，或在较小程度上由Co²⁺或Fe²⁺满足。对于XL-I和XL-II，过量未络合的Mg²⁺或Mn²⁺会降低速率；Mn²⁺的最佳浓度与测定中ATP的浓度大致相同，Mg²⁺的最佳浓度约为测定中ATP浓度的两倍。这与二价阳离子需要与ATP络合的概念以及这两种二价阳离子与ATP络合物的已知稳定常数一致。未络合的二价阳离子不抑制XL-III。未络合的ATP是XL-I和XL-II的中度抑制剂，是XL-III的弱抑制剂。数据表明，体内苯甲酸酯结合依赖于K⁺和Mg²⁺，并且阳离子效应复杂，与XL-III相比，XL-I和XL-II的阳离子效应有所不同。
