Petronini P G, Piedimonte G, Borghetti A F
Biochim Biophys Acta. 1982 Dec 8;693(1):13-21. doi: 10.1016/0005-2736(82)90465-5.
The rate of transport of phenylalanine by System L has been measured in SV40 3T3 cells at various cell densities. When the activity of the L system was determined before any cell depletion of intracellular amino acids, a density-dependent increase in transport paralleled the decrease in cell density. This regulation was lost after cell depletion but reappeared after reloading the cells with pertinent substrates of System L. The phenylalanine transport activity modulated by cell density appeared to be related to the internal level of amino acids capable of exchange up to a definite concentration, beyond which transport activity by System L did not parallel a further increase of internal substrate level. Analysis of the relationship between influx and substrate concentration suggested that two saturable components contribute to entry of phenylalanine and leucine in depleted and in reloaded cells: a low-affinity and a high-affinity component. Both kinetic parameters of the high-affinity component appeared to be modulated by the loading treatment, but only V changed markedly. Activation energies for the high-affinity component of the amino acid transport reaction were calculated from an Arrhenius plot in reloaded cells, and were found to be different for low- and high-density cultures. This result is consistent with the interpretation that cell density modulated the rates at which the amino acid-carrier complex can move within the cell membrane.
在不同细胞密度下，已对SV40 3T3细胞中系统L转运苯丙氨酸的速率进行了测定。当在细胞内氨基酸未耗尽之前测定L系统的活性时，转运的密度依赖性增加与细胞密度的降低平行。细胞耗尽后这种调节作用丧失，但在用系统L的相关底物重新加载细胞后又重新出现。受细胞密度调节的苯丙氨酸转运活性似乎与能够交换的氨基酸的内部水平有关，直至达到一定浓度，超过该浓度后，系统L的转运活性不再与内部底物水平的进一步增加平行。对流入与底物浓度之间关系的分析表明，两个可饱和成分有助于苯丙氨酸和亮氨酸进入耗尽的和重新加载的细胞：一个低亲和力成分和一个高亲和力成分。高亲和力成分的两个动力学参数似乎都受加载处理的调节，但只有V有明显变化。根据重新加载细胞中的阿累尼乌斯图计算氨基酸转运反应高亲和力成分的活化能，发现低密度和高密度培养物的活化能不同。这一结果与细胞密度调节氨基酸 - 载体复合物在细胞膜内移动速率的解释一致。
