Hoffman J L
Biochemistry. 1986 Jul 29;25(15):4444-9. doi: 10.1021/bi00363a041.
The chirality of biologically active S-adenosyl-L-methionine (AdoMet) is S,S, where the designations refer to the sulfur and the alpha-carbon, respectively. This paper describes a cation-exchange high-performance liquid chromatographic (HPLC) method for separating (S,S)-AdoMet from the biologically inactive (R,S)-AdoMet that results from racemization at the sulfur. This method was used to measure the rates of the degradation reactions of (S,S)-AdoMet as a function of pH. These reactions and the first-order rate constants, which were found at 37 degrees C and pH 7.5, are racemization, 1.8 X 10(-6) s-1; cleavage to homoserine lactone and 5'-(methylthio)adenosine, 4.6 X 10(-6) s-1; and hydrolysis to adenine and S-pentosylmethionine, 3 X 10(-6) s-1. Racemization showed no change in rate over the pH range from 7.5 to 1.5. The cleavage reaction persisted until the pH was lowered to 1.5, but hydrolysis ceased at pH 6. Commercial samples of nonradioactive AdoMet contained 20-30% (R,S)-AdoMet, while a sample of [methyl-3H]AdoMet had less than 1% (R,S)-AdoMet. Preparing enzyme substrates by mixing such samples will cause an underestimate of specific activity and an overestimate of the amount of product. The (R,S)-AdoMet/(S,S)-AdoMet ratio in mouse liver was 0.03, much less than the value of 0.19 calculated from the above rate constants. An enzyme extract from mouse liver did not degrade (R,S)-AdoMet, but a more thorough search may find such an activity. In any event, the cleavage and hydrolysis reactions partially balance the racemization of (S,S)-AdoMet in vivo and prevent excessive accumulation of (R,S)-AdoMet.
具有生物活性的S-腺苷-L-甲硫氨酸（AdoMet）的手性为S,S，其中这两个指定分别指硫原子和α-碳原子。本文描述了一种阳离子交换高效液相色谱（HPLC）方法，用于从因硫原子外消旋化而产生的无生物活性的（R,S）-AdoMet中分离出（S,S）-AdoMet。该方法用于测量（S,S）-AdoMet降解反应速率随pH的变化。这些反应以及在37℃和pH 7.5时发现的一级速率常数分别为：外消旋化，1.8×10⁻⁶ s⁻¹；裂解为高丝氨酸内酯和5'-（甲硫基）腺苷，4.6×10⁻⁶ s⁻¹；水解为腺嘌呤和S-戊糖基甲硫氨酸，3×10⁻⁶ s⁻¹。在pH值从7.5到1.5的范围内，外消旋化速率没有变化。裂解反应一直持续到pH降至1.5，但水解在pH 6时停止。非放射性AdoMet的商业样品含有20 - 30%的（R,S）-AdoMet，而[甲基-³H]AdoMet样品中（R,S）-AdoMet含量低于1%。通过混合此类样品制备酶底物会导致对比活性的低估和产物量的高估。小鼠肝脏中（R,S）-AdoMet/（S,S）-AdoMet的比值为0.03，远低于根据上述速率常数计算出的0.19的值。小鼠肝脏的酶提取物不会降解（R,S）-AdoMet，但更全面的研究可能会发现这样的活性。无论如何，裂解和水解反应在体内部分平衡了（S,S）-AdoMet的外消旋化，并防止了（R,S）-AdoMet的过度积累。
