Transport of Small Aliphatic Amines by Polyspecific Solute Carriers: Deciphering Structure-Function Relationships

Membrane proteins of the solute carrier (SLC) 22A and 47A families are polyspecific transporters critical for handling diverse endogenous and exogenous compounds, including many clinically relevant drugs. However, their substrate specificity remains poorly understood. To address this, we conducted a structure-function relationship analysis focusing on small aliphatic amines (alkylamines and alkanolamines) and enantioselectivity of their transport. Using HEK293 cells stably overexpressing organic cation transporters (OCT) 1, 2, or 3 (SLC22A1, -2, or -3) or multidrug and toxin extrusion transporters (MATE) 1 or 2-K (SLC47A1 and -2), substrate transport was quantified through liquid chromatographytandem mass spectrometry, with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate derivatization as needed. While most tested compounds exhibited physicochemical properties consistent with typical OCT and MATE substrates, compounds with a molecular weight of less than 145 Da were mostly not transported by OCT1, OCT3, or the MATEs. However, in great contrast, OCT2 demonstrated robust transport of small aliphatic amines and alkanolamines (molecular weight of 60-145 Da), with modest stereoselectivity favoring (S)-enantiomers. Structural complexity, such as carbon chain length and amino group positioning, strongly influenced transport activity and kinetics, while compounds with more than two positive charges at different positions were not transported. Additionally, we identified a novel role for OCT2 as an efficient efflux transporter for ethanolamine. These findings reveal critical molecular determinants underlying SLC-mediated transport, enhancing our understanding of OCT and MATE substrate preferences and mechanisms. This knowledge provides a foundation for better predicting transporter interactions and optimizing drug design.