Renal reabsorption of amino acids in the proximal tubule is achieved by a concerted action of several amino acid transporting systems. Recently a membrane transporter for the imino acid proline SIT1 (Sodium / Imino-acid Transporter, SLC6A20) a member of the Na+ and Cl--dependent neurotransmitter family was identified as a candidate for the classical System IMINO in kidney and for hereditary iminoacidurias in man.

In exploring OK cells as a tool for imino acid transport we have studied SIT1 expression in OK cells by PCR and proline tracer uptake experiments. By PCR we have amplified the mRNA of a homologues SIT1 gene from opossum. Proline transport in OK monolayers was Na+-dependent, Cl-sensitive and shows a high affinity (KM ~ 50-70 ?M). In competition assays, uptake was dose-dependently reduced by the non-labelled SIT1 substrates, e.g. hydroxyproline, N-methylproline, pipecolate and MeAIB. Other hydrophobic amino acids such as leucine and phenylalanine are also substrates for SIT1 but display markedly reduced affinities. A PAT-like proton-dependent uptake of proline could not be detected in OK cells. In conclusion a SIT1 transport protein is expressed in OK cells and L-proline transport shows all properties of System IMINO in kidney which makes these cells an appropriate model to investigate hereditary iminoaciduria.