Characterization of the glycine transport system GLYT 1 in human placental choriocarcinoma cells (JAR)

Abstract

Transport of glycine in confluent monolayer cultures of JAR human placental choriocarcinoma cells was investigated. Glycine uptake in these cells was made up of two components, one being Na +-dependent with no requirement for Cl − and the other being dependent on Na + as well as Cl −. Substrate specificity studies indicated that distinct transport systems were responsible for these two components. Alanine inhibited the Na +-dependent glycine uptake preferentially and the Na +- and Cl −-dependent glycine uptake represented > 95% of total uptake in the presence of 5 mM alanine. Competition experiments revealed that the Na +- and Cl −-dependent transport system exhibited a very narrow substrate specificity with affinity toward only glycine and its derivatives such as sarcosine, glycine methyl ester and glycine ethyl ester. These characteristics identify the transport system as GLYT 1. This system showed high affinity for glycine, with a Michaelis-Menten constant of 15 μM. The Na +:Cl −: glycine stoichiometry appeared to be 2:1:1. Treatment of JAR cells with calmodulin antagonists resulted in the inhibition of the transport function of GLYT 1 and this inhibition was solely due to a decrease in the maximal velocity of the system with no change in the substrate affinity. It is concluded that the placental choriocarcinoma cell line JAR expresses robust activity of the glycine transporter GLYT 1 and that the activity of this transporter is under the regulation of calmodulin-dependent cellular processes.