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

Transport of lysine by microvillous membranes was investigated by characterization of L-[H-3]lysine uptake in membrane vesicles isolated from human placentas. At least one Nat-independent system was observed at 22 degrees C and two systems at 37 degrees C. Lysine concentration dependence 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 with 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 degrees C. In the presence of N-ethylmaleimide, the uptake (37 degrees C) data 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 inhibited completely by L-arginine, L-histidine, and L-homoarginine. In the presence of Na+, uptake was inhibited completely by these same three amino acids and L-leucine but only partially by other neutral amino acids. To compare directly microvillous and basal membrane from the same 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 membranes. However, L-homoserine, L-alanine, and L-phenylalanine over a wide concentration range inhibited substantially less in microvillous (at both temperatures) than in basal membrane. Thus the y(+) system is common to the two membranes of the human placenta, but the functional characteristics of the transporters differ between them. In addition, the microvillous membrane possesses a second system that responds to temperature. The role of the temperature-sensitive, high-affinity, low-capacity system remains to be determined.