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Synthesis and Myocardial Kinetics of N-13 and C-11 Labeled Branched-Chain L-Amino Acids

University of California, Los Angeles, Los Angeles
Scripps Clinic and Research Foundation, La Jolla, California

Correspondence: For reprints contact: J. R. Barrio, PhD, UCLA School of Medicine, Dept. of Radiological Sciences, Div. of Biophysics, Los Angeles, CA 90024.

ABSTRACT

Glutamate dehydrogenase (GDH), immobilized on CNBr-activated Sepharose supports, was used with N-13 ammonia to aminate -ketoisocaproic acid (KIC), and -ketoisovaleric acid (KIV) to produce N-13-labeled branched-chain L-amino acids with radiochemical yields ranging from 29 % to 35 %. From kinetic and practical considerations, pH 7.5–8.0 was established to be optimal for the synthesis of N-13-labeled branched-chain-L-amino acids. Myocardial time-activity curves in dogs at control, during low-flow ischemia, reperfusion, and after transaminase inhibition following intracoronary bolus injection of the N-13-labeled amino acids were biexponential. Higher retention of N-13 activity was observed in ischemic segments both during low-flow ischemia (29.2 %) and reperfusion (23.2 %) when compared with controls (20.0%), (n = 4). On the other hand, transaminase inhibition decreased residue fractions from 21.0 % at control to 13.9 % (n = 4). The residual activity with L-[1-¹¹C]leucine allows for the calculation of protein synthesis rates.