The measurement of total lysine turnover in the rat by intravenous infusion of L-[U-14C] lysine

ABSTRACT

Rats were infused intravenously for periods up to 7 hr with L-[U-14C]lysine. After about 4 hr the specific activity of free lysine in the blood reached a plateau. From the value of the plateau specific activity it is possible to calculate the total lysine flux into or out of the blood stream. In normal rats the flux is equivalent to a total protein turnover of 25-30 g kg-1 day-1. In male, but not in female rats the total flux decreased significantly with increasing body weight or age. Six weeks of protein depletion caused a decrease of 30 percent in the flux. Alloxan diabetes in a small number of rats caused no significant change. After the first « hr of the infusion the specific activity of the mixed serum proteins increased almost linearly. From the rate of increase and the specific activity of the free lysine at plateau, an approximate estimate can be made of the renewal rate of the serum proteins. This estimate must be too low, because the specific activity of the free amino acid at the site of synthesis, e.g. in the liver cell, must always be lower than in serum. The validity of the continuous infusion method is discussed in relation to other methods of measuring total amino acid or protein turnover.(AU)