This enables eIF2B to promote the formation
of the Met-tRNAi-40S ribosomal subunit complex
and to initiate mRNA translation (12. In our study, leucine
had no effect on the phosphorylation of eIF2a in
both the liver and skeletal muscle.
Studies using cell cultures have identified a positive
role of leucine in the regulation of the peptide-chain
elongation process, which is regulated by the eEF2 pathway
[31]. In this pathway, the phosphorylation of eEF2,
which results in suppression of the elongation process,
is tightly regulated by eEF2 kinase [13]. Insulin or
amino acid-induced activation of mTOR/S6K1 signaling
causes inactivation of this eEF2 kinase, followed by
dephosphorylation of eEF2. Anabolic stimuli decrease
eEF2 phosphorylation, enabling the elongation process
to occur [14]. Interestingly, in contrast to cell culture
studies, our data showed that leucine did not have any
effect on phosphorylation of eEF2. This finding suggests
that the elongation process is not a rate limiting step in
the leucine-induced stimulation of protein synthesis in
skeletal muscle of neonatal pigs.