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The activities of specific transport systems were determined before and after large portions of the surface membrane had been interiorized by phagocytosis of inert particles. In five separate transport systems in rabbit polymorphonuclear leukocytes (adenosine and two adenine transport systems) and alveolar macrophages (adenosine and lysine transport systems), the rate of transport was unaffected even after an estimated 35–50% of the membrane had been internalized. Studies of the kinetics of lysine and adenosine transport, exchange diffusion of lysine transport in alveolar macrophages, and the specificities of adenine transport in polymorphonuclear leukocytes indicate that the nature of the membrane transport systems is not altered by phagocytosis. Therefore the constancy of transport indicates that the number of carriers remains the same before and after phagocytosis. It was also shown that this constancy of transport did not depend on the introduction into the surface of new transport sites during phagocytosis. Therefore transport sites are preserved on the surface during the internalization of membrane which accompanies phagocytosis. The results are best explained by the concept that the membrane is mosaic in character with geographically separate transport and phagocytic sites.

Trypsinization of alveolar macrophages depresses lysine transport but has no effect on nucleoside transport. The depression of the initial rate of lysine transport is due to a decreased rate of exchange diffusion presumably secondary to lowered intracellular pools of amino acids. It is shown directly that trypsinization accelerates the efflux of amino acids but not certain other metabolites, and reduces the capacity of the cell to concentrate amino acids.

Amino acid transport in the rabbit alveolar macrophage was studied with a rapid sampling technique which made possible the determination of initial rates of uptake. Lysine was transported by a single saturable system, designated AM-1 with a K m of 0.1 mM and a v max of 0.44 nmole/45 sec per 106 cells. Competition studies revealed that almost all natural amino acids had some affinity for the carrier, though it varied by several orders of magnitude. In general, there is preferential affinity for amino acids with a long lipolytic side chain and a terminal positive charge. Na+ and K+ inhibited lysine transport. A kinetic study of the Na+ inhibition revealed that its effect was due to depression of the v max , K m remaining constant (noncompetitive kinetics). Leucine was transported by the AM-1 system and at least one other system.