Is methionine transaminated in skeletal muscle?

Abstract

Methionine transamination is extensive in rat and chick skeletal-muscle homogenates, but is barely detectable in intact rat, but not chick, skeletal muscles. Branched-chain amino acids essentially block methionine transamination in intact muscles and homogenates from both species. The physiological significance of methionine transamination in skeletal muscle is questioned.

Methionine transamination and glutamine transaminases in skeletal muscle

“…Our results are not consistent with Cooper's theoretical concept that low glutamine transaminase activity in skeletal muscle may favour methionine degradation via a transamination pathway [1]. Although methionine is extensively transaminated in intact chick skeletal muscle incubated in the presence of 0.2-0.5 mM-methionine but in the absence of other amino acids [2], glutamine transaminase K and L activities are only barely detected in this tissue (Table 1). On the other hand, rat skeletal muscles contain considerable glutamine transaminase K and L activity, while methionine transamination is negligible or absent in intact muscles incubated in the absence of plasma concentrations of amino acids (Table 1).…”

“…These results suggest that glutamine transaminases K and L do not play an important role in methionine degradation in skeletal muscle. It is clear from our studies that little or no 2-oxo-4methylthiobutyrate is produced or accumulated in intact rat or chick skeletal muscles in the presence of plasma concentrations of amino acids [2]. Even though methionine is extensively transaminated in intact chick skeletal muscles incubated in the presence of 0.2-0.5 mM-methionine and in the absence of other amino acids, we found that only a very small amount of the 2-oxo-4methylthiobutyrate produced undergoes oxidative decarboxylation [2].…”

“…They also reported the transamination of 5 mM-2-oxo-4methylthiobutyrate with 40 mM-L-albizziin in rat skeletal muscle, catalysed by glutamine transaminase L [5]. However, they did not measure the transmination of methionine itself with a-oxo acids in skeletal muscle extracts [5] as cited in our recent paper [2].…”

“…Glutamine transaminase K was assayed in the presence of 10 mM-glutamine, 50,aM-phenylpyruvate and 50 mM-sodium borate (pH 8.5) as described by Cooper & Meister [3][4][5], except that phenylalanine was measured by h.p.l.c. [2]. Glutamine transaminase L was assayed in the presence of 10 mM-Lalbizziin, 50 /iM-2-oxo-4-methylthiobutyrate and 50 mmsodium borate (pH 8.5) as described by Cooper & Meister [3][4][5], except that methionine was measured by h.p.l.c.…”

“…Glutamine transaminase L was assayed in the presence of 10 mM-Lalbizziin, 50 /iM-2-oxo-4-methylthiobutyrate and 50 mmsodium borate (pH 8.5) as described by Cooper & Meister [3][4][5], except that methionine was measured by h.p.l.c. [2]. Total activity of the enzyme is expressed as the cytosolic plus mitochondrial activity.…”