THE H-3 LEUCINE TRACER - ITS USE IN KINETIC-STUDIES OF PLASMA-LIPOPROTEINS

H-3-leucine administered as a bolus has been widely used as a tracer i n kinetic investigations of protein synthesis and secretion. After int ravenous injection, plasma specific radioactivity decays over several orders of magnitude during the first half-day, followed by a slow deca y lasting a number of weeks that results from recycling of the leucine tracer as proteins are degraded and H-3-leucine reenters the plasma p ool, In studies in which kinetic data are analyzed by mathematical com partmental modeling, plasma leucine activity is generally used as a fo rcing function to drive the input of H-3-leucine into the protein synt hesis pathway. H-3-leucine is an excellent tracer during the initial h ours of rapidly decreasing plasma activity; thereafter, reincorporatio n of recycled tracer into new protein synthesis obscures the tracer da ta from proteins with slower turnover rates. Thus, for proteins such a s plasma albumin and apolipoprotein (apo) A-I, this tracer is unsatisf actory for measuring fractional catabolic (FCR) and turnover rates. By contrast, the kinetics of plasma very-low-density lipoprotein (VLDL)- apoB, a protein with a residence time of approximately 5 hours, are re adily measured, since kinetic parameters of this protein can be determ ined by the time plasma leucine recycling becomes established. However , measurement of VLDL-apoB specific radioactivity extending up to 2 we eks provides further data on the kinetic tail of VLDL-apoB. Were plasm a leucine a direct precursor for the leucine in VLDL-apoB, the kinetic s of the plasma tracer should determine the kinetics of the protein. H owever, this is not the case, and the deviations from linearity are in terpreted in terms of (1) the dilution of plasma leucine in the liver by unlabeled dietary leucine; (2) the recycling of hepatocellular leuc ine from proteins within the liver, where recycled cellular leucine do es not equilibrate with plasma leucine; and (3) a ''hump'' in the kine tic data of VLDL-apoB, which we interpret to reflect recycling or rete ntion of a portion of the apoB protein within the hepatocyte, with its subsequent secretion. Because hepatocellular tRNA is the immediate pr ecursor for synthesis of these secretory proteins, its kinetics should be used as the forcing function to drive the modeling of this system. The VLDL-apoB tail contains the information needed to modify the plas ma leucine data, to provide an appropriate forcing function when using H-3-leucine as a tracer of apolipoprotein metabolism. This correction is essential when using H-3-leucine as a tracer for measuring low-den sity lipoprotein (LDL)-apoB kinetics. The H-3-leucine tracer also high lights the importance of recognizing the difference between plasma and system residence times, the latter including the time the tracer resi des within exchanging extravascular pools. The inability to determine these fractional exchange coefficients for apoA-I and albumin explains the failure of this tracer in kinetic studies of these proteins. For apoB-containing lipoproteins, plasma residence times are generally det ermined, and these measurements can be made satisfactorily with H-3-le ucine. Copyright (C) 1997 by W.B. Saunders Company.