AMINO-ACID (SYSTEM-A) TRANSPORTER ACTIVITY IN MICROVILLOUS MEMBRANE-VESICLES FROM THE PLACENTAS OF APPROPRIATE AND SMALL-FOR-GESTATIONAL-AGE BABIES

Although a number of causes of poor fetal growth are known, the involv ement of placental transport proteins in the etiology of growth retard ation is not understood. The aim of this study was to investigate the activity of the system A amino acid transporter and the Na+/H+ exchang er in vesicles isolated from the microvillous membrane of the syncytio trophoblast of placentas of appropriate and small for gestational age babies. There were no biochemical differences between the membranes fr om the two groups of placentas, and there nas no difference in the act ivity of the Na+/H+ exchanger. The initial rate of uptake of methylami noisobutyric acid (a nonmetabolizable amino acid analogue) was 63% low er in vesicles from placentas of small for gestational age babies. Kin etic analysis of the system A transporter (utilized by methylaminoisob utyric acid) showed that the V-max in the vesicles from placentas of s mall for gestational age babies (0.24 +/- 0.03 nmol/mg protein)30 s, n = 5) wits significantly lower than that in vesicles from placentas of appropriate for gestational age babies (0.64 +/- 0.09 nmol)mg protein )30 s, n = 4, p < 0.001), whereas the Km was not different between the two groups. It is concluded that there is an abnormality of system A amino acid transporter function in placentas of small for gestational age babies.