Kinetics of amino acid transport across bone marrow cell membranes.

Dog bone marrow nucleated cells were incubated in media containing labeled L-amino acids, and the cellular accumulation of radioactivity as a function of time was measured and analyzed according to a three-compartment model.(a) The turnover half-time of intracellular histidine arising from extracellular sources was 6.0 +/-0.7 (SEM) min. Similar turnover half-time for serine was 10 +/-2 (SEM) min; for tryptophan, 6.5 +/-1.2 (SEM) min; and for methionine, 4.4 +/-0.6 (SEM) min. Loss of the intracellular amino acids to the extracellular space accounted for the major portion of their turnover.(b) Each of the four amino acids noted above appeared to be actively transported into the cell.(c) At physiologic extracellular histidine concentrations, histidine entered the cell predominantly by a facilitated process with an apparent Michaelis constant of 0.28 mmole/liter and a limiting flux of 14 x 10(-8) mmumole/min per cell. Loss of histidine from the cell appeared to be substantially facilitated with an apparent Michaelis constant greater than that for histidine entry.(d) Insulin and glucagon had no measurable effect on histidine transport across the bone marrow cell membrane.(e) Methionine depressed the influx and the fractional turnover rate of the intracellular pool of both histidine and serine.(f) The extent of cellular accumulation of alpha-N-formiminoglutamate and alpha-N-formylglutamate was about 1/100 that of histidine. alpha-N-formiminoglutamate added to the culture was about (1/4) as effective as histidine in providing monocarbon fragments for DNA thymine synthesis.