Abstract
Guanidinoacetic acid (GAA) is the biosynthetic precursor of creatine which is involved in storage and transmission of phosphate-bound energy. Hepatocytes readily convert GAA to creatine, raising the possibility that the active uptake of GAA by hepatocytes is a regulatory factor. The purpose of this study is to investigate and identify the transporter responsible for GAA uptake by hepatocytes. The characteristics of [14C]GAA uptake by hepatocytes were elucidated using the in vivo liver uptake method, freshly isolated rat hepatocytes, an expression system of Xenopus laevis oocytes, gene knockdown, and an immunohistochemical technique. In vivo injection of [14C]GAA into the rat femoral vein and portal vein results in the rapid uptake of [14C]GAA by the liver. The uptake was markedly inhibited by γ-aminobutyric acid (GABA) and nipecotinic acid, an inhibitor of GABA transporters (GATs). The characteristics of Na+- and Cl−-dependent [14C]GAA uptake by freshly isolated rat hepatocytes were consistent with those of GAT2. The Km value of the GAA uptake (134 µM) was close to that of GAT2-mediated GAA transport (78.9 µM). GABA caused a marked inhibition with an IC50 value of 8.81 µM. The [14C]GAA uptake exhibited a significant reduction corresponding to the reduction in GAT2 protein expression. GAT2 was localized on the sinusoidal membrane of the hepatocytes predominantly in the periportal region. This distribution pattern was consistent with that of the creatine biosynthetic enzyme, S-adenosylmethionine∶guanidinoacetate N-methyltransferase. GAT2 makes a major contribution to the sinusoidal GAA uptake by periportal hepatocytes, thus regulating creatine biosynthesis in the liver.
Highlights
Guanidinoacetic acid (GAA) is the biosynthetic precursor of creatine which plays an important role in storage and transmission of phosphate-bound energy in tissues with high energy demands [1]
GABA, taurine, and creatine inhibited the [14C]GAA uptake in a concentration-dependent manner with an IC50 value of 8.8161.33 mM, 1.1360.30 mM, and 1.4260.53 mM, respectively (Figure 2C–E). These results indicate that Na+- and Cl2dependent GAA uptake by hepatocytes is mediated via a carrier system preferring GABA over taurine and creatine, most probably GABA transporters (GATs)
The present study demonstrates that GAT2 is responsible for the rapid uptake of GAA on the sinusoidal membrane of hepatocytes predominantly in the periportal region
Summary
Guanidinoacetic acid (GAA) is the biosynthetic precursor of creatine which plays an important role in storage and transmission of phosphate-bound energy in tissues with high energy demands [1]. The physiological importance of creatine biosynthesis has been demonstrated by an inherited deficiency of the creatine biosynthetic enzyme, S-adenosylmethionine:guanidinoacetate Nmethyltransferase (GAMT) [2]. Patients exhibit a severe reduction in creatine and the simultaneous accumulation of GAA in the plasma, and brain, causing mental retardation, delayed speech, and epilepsy [3]. High levels of GAMT mRNA are found in this tissue in humans and mice [4] and a severe reduction in GAMT activity has been detected in the liver of patients with GAMT deficiency [5]. It is conceivable that GAA needs to be delivered from the circulating blood to hepatocytes for creatine synthesis
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
