A multicomponent analysis of amino acid transport systems in human lymphocytes. 1. Kinetic parameters of the A and L systems and pathways of uptake of naturally occurring amino acids in blood lymphocytes

Abstract

We have determined the kinetic parameters of natural and system‐specific synthetic amino acid transport by human blood lymphocytes, using a multi‐component computer analysis that separates carrier‐mediated uptake from diffusion. These studies were initiated in order to provide the basis for studies of human blood T and B lymphocytes and malignant lymphocytes. Methylaminoisobutyric acid (methyl‐AIB) and 2‐amino‐2‐carboxy‐bicyclo (2,2,1) heptane (BCH) uptakes into lymphocytes were measured as prototypes of A‐ and L‐system amino acid transport. The Michaelis constant for methyl‐AIB uptake was 540 μM; the maximal velocity of uptake was 28 μmol/L cell water/min, and the diffusion coefficient was .004 min⁻¹. In contrast, the Michaelis constant for BCH uptake was 63 μM; the maximal velocity was 969 μmol/L cell water/min, and the diffusion coefficient was .141 min⁻¹. The transport of the naturally occurring amino acids, alanine, proline, and leucine was defined by studies of: (1) competitive inhibition with the system‐specific synthetic amino acids, methyl‐AIB and BCH, (2) the effect of the transcellular sodium gradient on transport, and (3) evaluation of the time‐dependent increase of transport in amino acid‐deficient medium (adaptation). Alanine was transported principally (∼70%) by the ASC‐system, and leucine was transported principally (70%) by the L‐system in lymphocytes. The analysis of proline transport was more complex because of a large component of uptake by diffusion even at low amino acid concentrations. Taken together, the kinetics of sodium‐sensitive uptake and the results of competitive inhibition studies indicated that proline was transported by the A‐system (30%), the ASC system (30%), and also by the L‐system (15%).