Characterization of a novel variant of amino acid transport system asc in erythrocytes from Przewalski's horse (Equus przewalskii)

In thoroughbred horses, red blood cell amino acid transport activity is Na+-independent and controlled by three codominant genetic alleles (h, l, s), coding for high-affinity system asc1 (L-alanine apparent Km for influx at 37 °C [Formula: see text] 0.35 mM), low-affinity system asc2 (L-alanine Km[Formula: see text] 14 mM), and transport deficiency, respectively. The present study investigated amino acid transport mechanisms in red cells from four wild species: Przewalski's horse (Equus przewalskii), Hartmann's zebra (Zebra hartmannae), Grevy's zebra (Zebra grevyi), and onager (Equus hemonius). Red blood cell samples from different Przewalski's horses exhibited uniformly high rates of L-alanine uptake, mediated by a high-affinity asc1-type transport system. Mean apparent Km and Vmax values (± SE) for L-alanine influx at 37 °C in red cells from 10 individual animals were 0.373 ± 0.068 mM and 2.27 ± 0.11 mmol (L cells∙h), respectively. As in thoroughbreds, the Przewalski's horse transporter interacted with dibasic as well as neutral amino acids. However, the Przewalski asc1 isoform transported L-lysine with a substantially (6.4-fold) higher apparent affinity than its thoroughbred counterpart (Km for influx 1.4 mM at 37 °C) and was also less prone to trans-stimulation effects. The novel high apparent affinity of the Przewalski's horse transporter for L-lysine provides additional key evidence of functional and possible structural similarities between asc and the classical Na+-dependent system ASC and between these systems and the Na+-independent dibasic amino acid transport system y+. Unlike Przewalski's horse, zebra red cells were polymorphic with respect to L-alanine transport activity, showing high-affinity or low-affinity saturable mechanisms of L-alanine uptake. Onager red cells transported this amino acid with intermediate affinity (apparent Km for influx 3.0 mM at 37 °C). Radiation inactivation analysis was used to estimate the target size of system asc in red cells from Przewalski's horse. The transporter's in situ apparent molecular weight was 158 000 ± 2500 (SE).Key words: amino acid transport, erythrocyte, Przewalski's horse.