LYSINE UPTAKE BY HUMAN PLACENTAL MICROVILLOUS MEMBRANE - COMPARISON OF SYSTEM Y(+) WITH BASAL MEMBRANE

Transport of lysine by microvillous membranes was investigated by char acterization of L-[H-3]lysine uptake in membrane vesicles isolated fro m human placentas. At least one Nat-independent system was observed at 22 degrees C and two systems at 37 degrees C. Lysine concentration de pendence data were fit by a one- or two-system model with a Michaelis- Menten constant (K-m) of 124 +/- 28 mu M and a maximum velocity (V-max ) of 35.1 +/- 7.7 pmol . mg protein(-1)min(-1) at 22 degrees C and wit h K-m values of 1 +/- 0.6 and 245 +/- 51 mu M and V-max values of 0.14 +/- 0.07 and 45.8 +/- 8.7 pmol . mg protein(-1). 30 s(-1) at 37 degre es C. In the presence of N-ethylmaleimide, the uptake (37 degrees C) d ata were fit by a one-system model with kinetic parameters similar to the lower K, system. Uptake of L-lysine in the absence of Na+ was inhi bited completely by L-arginine, L-histidine, and L-homoarginine. In th e presence of Na+, uptake was inhibited completely by these same three amino acids and L-leucine but only partially by other neutral amino a cids. To compare directly microvillous and basal membrane from the sam e placenta, we examined the inhibition of 20 mu M lysine uptake in the presence of Nai. Inhibition by L-leucine was similar in the two membr anes. However, L-homoserine, L-alanine, and L-phenylalanine over a wid e concentration range inhibited substantially less in microvillous (at both temperatures) than in basal membrane. Thus the y(+) system is co mmon to the two membranes of the human placenta, but the functional ch aracteristics of the transporters differ between them. In addition, th e microvillous membrane possesses a second system that responds to tem perature. The role of the temperature-sensitive, high-affinity, low-ca pacity system remains to be determined.