Besterman J M, Airhart J A, Low R B, Rannels D E
J Cell Biol. 1983 Jun;96(6):1586-91. doi: 10.1083/jcb.96.6.1586.
Intracellular degradation of exogenous (serum) proteins provides a source of amino acids for cellular protein synthesis. Pinocytosis serves as the mechanism for delivering exogenous protein to the lysosomes, the major site of intracellular degradation of exogenous protein. To determine whether the availability of extracellular free amino acids altered pinocytic function, we incubated monolayers of pulmonary alveolar macrophages with the fluid-phase marker, [14C]sucrose, and we dissected the pinocytic process by kinetic analysis. Additionally, intracellular degradation of endogenous and exogenous protein was monitored by measuring phenylalanine released from the cell monolayers in the presence of cycloheximide. Results revealed that in response to a subphysiological level of essential amino acids or to amino acid deprivation, (a) the rate of fluid-phase pinocytosis increased in such a manner as to preferentially increase both delivery to and size of an intracellular compartment believed to be the lysosomes, (b) the degradation of exogenously supplied albumin increased, and (c) the fraction of phenylalanine derived from degradation of exogenous albumin and reutilized for de novo protein synthesis increased. Thus, modulation of the pinosome-lysosome pathway may represent a homeostatic mechanism sensitive to the availability of extracellular free amino acids.
外源性（血清）蛋白质的细胞内降解为细胞蛋白质合成提供了氨基酸来源。胞饮作用是将外源性蛋白质输送到溶酶体的机制，溶酶体是外源性蛋白质细胞内降解的主要场所。为了确定细胞外游离氨基酸的可用性是否会改变胞饮功能，我们用液相标记物[14C]蔗糖孵育肺泡巨噬细胞单层，并通过动力学分析剖析胞饮过程。此外，通过测量在环己酰亚胺存在下从细胞单层释放的苯丙氨酸来监测内源性和外源性蛋白质的细胞内降解。结果显示，在对生理水平以下的必需氨基酸或氨基酸剥夺作出反应时，（a）液相胞饮作用速率增加，其方式优先增加了被认为是溶酶体的细胞内区室的输送量和大小，（b）外源性供应白蛋白的降解增加，以及（c）源自外源性白蛋白降解并重新用于从头蛋白质合成的苯丙氨酸比例增加。因此，对胞饮体-溶酶体途径的调节可能代表了一种对细胞外游离氨基酸可用性敏感的稳态机制。
