Cartenì-Farina M, della Ragione F, Cacciapuoti G, Porcelli M, Zappia V
Biochim Biophys Acta. 1983 Jan 19;727(2):221-9. doi: 10.1016/0005-2736(83)90407-8.
The transport and metabolism of 5'-deoxy-5'-S-methylthioadenosine have been studied in intact human erythrocytes. The sulfur nucleoside is rapidly accumulated into red cells and the extent of uptake largely exceeds the theoretical equilibrium between inner and outer compartment owing to its conversion into a non-permeable compound, namely 5-methylthioribose 1-phosphate. To characterize the nucleoside transport, phosphate-depleted erythrocytes, in which the methylthioadenosine metabolism is negligible, have been employed. The results indicate that: (i) the transport occurs via a facilitated-diffusion mechanism; (ii) the process is not energy-dependent and (iii) no specific cation is required. The kinetic analyses of both the transport and the metabolism show that the uptake of methylthioadenosine is a result of the tandem action of a transport step of high capacity (Vmax = 604 +/- 51 pmol/10(6) cells per min) and low affinity (Km = 3270 +/- 321 microM) followed by a metabolic step of low capacity (Vmax = 6.6 pmol/10(6) cells per min) and high affinity (Km = 30 microM). Furthermore, a substrate inhibition exerted by methylthioadenosine at high concentration (over 200 microM) on its specific phosphorylase is reported for the first time. Experiments performed with several analogs of the thioether indicate that the adenine amino group and the hydrophobic substituent at the 5'-position are critical for the transport carrier recognition. Adenine is the most powerful inhibitor of methylthioadenosine transport.
在完整的人红细胞中研究了5'-脱氧-5'-S-甲基硫代腺苷的转运和代谢。这种硫代核苷迅速积累到红细胞中，由于其转化为一种不可渗透的化合物，即5-甲基硫代核糖1-磷酸，摄取程度大大超过了内外隔室之间的理论平衡。为了表征核苷转运，使用了磷酸缺乏的红细胞，其中甲基硫代腺苷的代谢可以忽略不计。结果表明：（i）转运通过易化扩散机制发生；（ii）该过程不依赖能量；（iii）不需要特定阳离子。转运和代谢的动力学分析表明，甲基硫代腺苷的摄取是一个高容量（Vmax = 604 +/- 51 pmol/10⁶个细胞每分钟）和低亲和力（Km = 3270 +/- 321 μM）的转运步骤与一个低容量（Vmax = 6.6 pmol/10⁶个细胞每分钟）和高亲和力（Km = 30 μM）的代谢步骤串联作用的结果。此外，首次报道了高浓度（超过200 μM）的甲基硫代腺苷对其特异性磷酸化酶产生底物抑制作用。用几种硫醚类似物进行的实验表明，腺嘌呤氨基和5'-位的疏水取代基对于转运载体识别至关重要。腺嘌呤是甲基硫代腺苷转运最有效的抑制剂。
