Measurement of muscle protein synthesis by positron emission tomography with L-[methyl-11C]methionine: effects of transamination and transmethylation.
BACKGROUND
Positron emission tomography with L-[methyl-11C]methionine provides a measure of regional protein synthesis rate (PSR) in skeletal muscle. However, the validity of the method depends on incorporation of methionine into protein with minimal transamination, transmethylation, or both. To test directly these assumptions, uptake of L-[methyl-14C]methionine in skeletal muscle was measured in control and cycloheximide-treated rats.


METHODS
Normal and cycloheximide-treated rats (n = 8/group) were injected with 50 microCi of L-[methyl-14C]methionine and arterial blood sampled over 90 minutes. After killing, thigh muscle was homogenized, centrifuged, and treated with trichloroacetic acid. PSR from circulating methionine was estimated from trichloroacetic acid-precipitable radioactivity, arterial time-activity curves, and plasma methionine concentrations.


RESULTS
In normal rats, approximately 70% of the tissue radioactivity was precipitated with trichloroacetic acid. In normal animals, PSR was 0.22 nmoles x min(-1) x g(-1), in excellent agreement with previous results. In the cycloheximide-treated group, PSR was 0.0032 nmoles x min(-1) x g(-1); approximately 98% reduction compared with controls.


CONCLUSIONS
These studies support the hypothesis that L-[methyl-11(14C]methionine accumulates in skeletal muscle as 11(14)C-labeled protein.
Carter
E A
EA
Department of Surgery, Massachusetts General Hospital, Harvard Medical School, Boston 02114, USA.
Yu
Y M
YM
Alpert
N M
NM
Bonab
A A
AA
Tompkins
R G
RG
Fischman
A J
AJ
D000586:Amination; D000818:Animals; D003513:Cycloheximide / Q000494:pharmacology; D008297:Male; D008715:Methionine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000493:pharmacokinetics; D008745:Methylation; D009124:Muscle Proteins / Q000096:biosynthesis*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D014055:Tomography, Emission-Computed / Q000379:methods*; D000637:Transaminases
D009124:Muscle Proteins; D011500:Protein Synthesis Inhibitors; D003513:Cycloheximide; D008715:Methionine; C038344:methionine methyl ester; D000637:Transaminases