Protein elongation rates in tissues of growing and adult sheep.

To identify the relative roles of translation initiation and elongation in the long term control of protein synthesis in ovine tissues, fractional synthesis rates (FSR) and ribosomal transit times (RTT) were measured in vivo in 24 ewe lambs at 3 levels of intake [maintenance (M), 1.5M, and 2M] and 8 mature ewes at 2M intake. After 17 to 25 d on treatment, animals were given an i.v. flooding dose of l-[ring-2,6-(3)H]phenylalanine and tissues were collected for analysis of radioactivity in free protein, total protein, and nascent ribosome-associated proteins. Ribosome transit time (the inverse of elongation rate) averaged 83, 393, 183, 241, 85, and 113 s for liver, duodenum, skin, rumen, semimembranosus, and LM, respectively. In response to an increased level of intake, protein FSR increased (P < 0.01) in all tissues except rumen and was attributed to greater translational efficiency. There was no effect (P > 0.50) of intake on RTT in these tissues, and the estimated proportion of ribosomes attached to and actively translating mRNA was increased (P < 0.07), indicating that an upregulation of initiation was responsible for the greater FSR. Mature ewes exhibited lower (P < 0.10) protein FSR in all tissues compared with lambs, which was related to a decline in the RNA:protein ratio in all tissues except for liver and duodenum. In all tissues but liver and semimembranosus, RTT increased (P < 0.10) with age. The lower elongation rate was not considered to have influenced the protein synthetic rate, but it caused an increase in the proportion of ribosomes actively translating mRNA. It is anticipated that this work will provide direction to future studies of the molecular mechanisms of chronic FSR control.