A New Assessment of In Vivo Enzyme Activities in Metbolic Disorders Using Stable Isotope Methodology

本文研究了L-组氨酸在人体内的药代动力学，以评价组氨酸解氨酶系统在L-组氨酸转化为尿刊酸中的作用。两名健康志愿者（受试者A和B）单次口服100 mg L-[3，3-^2H_2，1 '，3'-<15>^N_2]组氨酸。在给药后24小时内采集血液和尿液样品，并通过稳定同位素稀释质谱法进行分析。根据二室模型计算药代动力学参数。受试者A（t_&lt;1/2&gt;=1.0hr）的标记L-组氨酸消除速度比受试者B（t_&lt;1/2&gt;=1.9hr）快约2倍。受试者A和受试者B的总体清除率（CL_T）分别为70.0升/小时和30.0升/小时。肾脏清除率与总体清除率的比值较低（CL_<re>/CL_T ;受试者A为1.04%，受试者B为0.43%），表明大部分L-组氨酸通过非肾脏过程消除。L-组氨酸迅速代谢为尿刊酸。受试者A的尿刊酸最大血浆浓度在30分钟时为59.61 ng/ml，受试者B的尿刊酸最大血浆浓度在60分钟时为46.10 ng/ml。尿刊酸的尿排泄率与原型L-组氨酸的血浆浓度的曲线的斜率被证明反映了L-组氨酸对尿刊酸的代谢清除率。本研究中讨论的体内人组氨酸解氨酶活性的评价方法对于组氨酸血症异质性的生化和临床阐明具有重要价值。

The pharmacokinetics of _L-histidine in human has been investigated to evaluate the in vivo histidine ammonia-lyase system for the conversion of _L-histidine to urocanic acid. Two healthy volunteers (subjects A and B) received a single 100-mg oral dose of _L-[3,3-^2H_2,1', 3'-^<15>N_2] histidine. Blood and urine samples were obtained over 24 hr after the administration and analyzed by stable isotope dilution mass spectrometry. The pharmacokinetic parameters were calculated based on a two-compartment model. The labeled _L-histidine in subject A (t_<1/2>=1.0hr) was eliminated approximately twice faster than that in subject B (t_<1/2>=1.9hr). The total body clearances (CL_T) were 70.0 liters/hr in subject A and 30.0 liters/hr in subject B.The low ratios of the renal clearance to the total body clearance (CL_<re>/CL_T ; 1.04% for subject A and 0.43% for subject B) indicated that most of _L-histidine was eliminated via the non-renal processes._L-Histidine was rapidly metabolized to urocanic acid. The maximum plasma concentrations of urocanic acid were 59.61 ng/ml at 30 min for subject A and 46.10 ng/ml at 60 min for subjectB.The slope of the plot of urinary excretion rate of urocanic acid vs.the plasma concentration of unchanged _L-histidine was demonstrated to reflect the metabolic clearance of _L-histidine to urocanic acid. The method of evaluating the in vivo human histidine ammonialyase activities discussed in this study offers a significant value with regard to the biochemical and clinical elucidations of the heterogeneity of histidinemia.