Peter J. Garlick

The effect of protein deprivation and starvation on the rate of protein synthesis in tissues of the rat

1. The fractional rate of protein synthesis was measured in tissues of rats in vivo by continuous infusion of [14C]tyrosine. In growing animals proteins of liver and kidney were renewed at a rate greater than 50% per day, those in skeletal muscle, brain and heart at a rate between 13 and 23% per day. 2. Protein synthesis was also measured in liver, kidney, heart, brain and skeletal muscle of rats either given a protein-free diet for 21 days or starved for 2 days. During the first 2 days no clear differences between the effects of these two regimes could be detected. 3. Gastrocnemius muscle did not lose tissue protein till after 9 days without protein in the diet. The rate of protein synthesis was halved after 1 day and halved again after 21 days without protein. It was deduced that the rate of protein breakdown in muscle had declined also. 4. In liver the loss of protein was immediate without any apparent change in the fractional rate of protein synthesis. Between 2 and 21 days of dietary protein deprivation the liver lost protein slowly but the fractional rate of protein synthesis was increased. It is proposed that lack of protein in the diet also causes an increase in the rate of liver protein breakdown.

The effect of unilateral phrenicectomy on the rate of protein synthesis in rat diaphragm in vivo

The fractional rate of protein synthesis (ks) in the denervated rat-diaphragm has been measured in vivo by the continuous amino acid infusion technique at 1, 3, 5 and 10 days after nerve section, and compared with the rate determined in normal rats. Similar rates of protein synthesis, 14% per day, were found for both the left and right hemidiaphragms in the control animals. In the denervated rats, the rates of protein synthesis in the contralateral control hemidiaphragms were significantly increased as soon as 1 day after nerve section. This is considered to be evidence of a compensatory synthesis in the control tissues. In the denervated hemidiaphragm, the rate of protein synthesis had doubled by the third day after nerve section, but by the fifth day had fallen slightly to a value some 50% greater than that of the controls, and remained at this level for a further 5 days. Based on these measured values of protein synthetic rate, calculated estimates have been made of the rate of protein degradation necessary to account for the reported (Turner, L.V. and Manchester, K.L. (1972) Biochem. J. 128, 789-801) changes in mass of the denervated tissue. During the first three days after nerve section, the rate constant for degradation increased to more than twice the normal rate for skeletal muscle, and remained at this value throughout the peak of the hypertrophy.

ASSESSMENT OF PROTEIN METABOLISM IN THE INTACT ANIMAL

Methods of quantifying whole-body protein turnover were developed for measurements in human subjects, but can also be valuable tools for the study of nutrition or growth in experimental animals. Comparison between species indicates that the relationship between the rate of whole-body protein turnover and body weight is similar to that between fasting metabolic rate and body weight. Three techniques have been described using both 14 C-and 15 N-labelled amino acids. All give similar rates under normal dietary conditions and respond in the same way to changes in diet and growth rate. In animals the constant infusion of a 14 C-labelled amino acid will probably be found to be the most accurate and convenient method. Constant infusion also has the advantage of being the method of choice for studies of protein synthesis and breakdown in individual tissues of larger animals. The rate of synthesis is obtained by removing each tissue at the end of the infusion for measurements of the specific activity of the free and protein-bound amino acid. The rate of breakdown can also be derived if, in addition to synthesis, the growth rate of the tissue protein mass is also measured.

METHODS FOR DETERMINING PROTEIN TURNOVER

Publisher Summary The conventional method for measuring the rate of protein breakdown in vivo involves injection of a labeled protein precursor and measurement of the rate of decay of labeled protein. This chapter describes an alternative method for estimating the breakdown by measuring the rate of synthesis and the rate of change of protein mass. Measurement of the rate of synthesis from the incorporation of labeled amino acids into protein is not affected by the heterogeneity of turnover rates in whole tissues, as long as incorporation is measured over a relatively short period. Breakdown should be estimated indirectly as the difference between the measured rate of protein synthesis and the net rate of growth of protein mass. This chapter also discusses the sources of errors in the indirect method of calculation of breakdown from the difference between synthesis and growth. The change in muscle protein is surprisingly difficult to measure accurately because of its slow rate.