Acetylation of L-leucine switches its carrier from the L-amino acid transporter (LAT) to organic anion transporters (OAT)

ABSTRACT

N-acetyl-DL-leucine is an analogue of the alpha amino acid leucine with a chiral stereocenter. The active L-enantiomer of the racemate is currently under development for rare neurological disorders. Here we present evidence that a selective recognition of N-acetyl-L-leucine versus L-leucine by different uptake transporters significantly contributes to the therapeutic effects of N-acetyl-L-leucine. A previous study of the pharmacokinetics of racemic N-acetyl-DL-leucine and N-acetyl-L-leucine revealed D-L enantiomer competition and saturation kinetics, best explained by carrier-mediated uptake. The strategy we used was to first analyze the physicochemical properties associated with good oral bioavailable drugs and how these are alerted by N-acetylation by comparing N-acetyl-L-leucine with L-leucine. Using in silico computational chemistry we found that N-acetylation has a profound impact on certain physicochemical properties that can rationalize why N-acetyl-L-leucine is drug-like compared to L-leucine. Our calculations show that at physiological pH, L-leucine is a zwitterion, whereas N-acetyl-L-leucine is present as mainly an anion. Specifically, N-acetylation removes a charge from the nitrogen at physiological pH and N-acetyl-L-leucine is an anion that is then a substrate for the organic anion transporters. We examined N-acetyl-L-leucine uptake in human embryonic kidney cells overexpression candidate organic anion transporters (OAT) and pharmacological inhibitors. We found that N-acetyl-L-leucine is a translocated substrate for OAT1 and OAT3 with low affinity (Km ~10 mM). In contrast, L-leucine is known to be transported by the L-type Amino Acid Transporter (LAT) with high affinity (Km ~0.2 mM) and low capacity. The clinical consequence is that L-leucine uptake becomes saturated at 50-fold lower concentration than N-acetyl-L-leucine. These results demonstrate a mechanism of action that explains why N-acetyl-L-leucine is effective as a drug and L-leucine itself is not.