THE TURNOVER RATE OF SERUM ALBUMIN IN MAN AS MEASURED BY I131-TAGGED ALBUMIN 1

Abstract

The turnover rate of human albumin was studied by following the disappearance of intraven. admd. I131 tagged albumin in 1-10 mg. doses without effect on the subject''s metabolism. Albumin labelled with 2-3 iodine atoms/molecule remained tagged after intraven. injn. in rabbits and behaved like native protein by immunochemical tests. Human subjects received Lugol''s soln. preceding and throughout studies to inhibit thyroid uptake and promote excretion of radioiodine liberated on degradation of admd. iodo-albumin. On adequate dietary intake the serum albumin concn. and body wt. remained relatively stable. Steady state conditions were assumed and the turnover rate was considered to signify rates of synthesis and degradation. The exponential fall in plasma radioactivity after the 2d day was considered to represent the rate of degradation of the admd. iodo-albumin, or rate of replacement of tagged by untagged albumin, hence the turnover rate of the protein. The "exchangeable albumin pool" was calculated by the isotope dilution principle. The product of the exchangeable albumin pool and turnover rate yielded turnover of albumin in g./day. In 21 normal male medical students the following mean values were obtained: exchangeable albumin pool, 259 [plus or minus] 40 grams and 232 [plus or minus] 34 grams/1.73 sq. m of surface area; half-time, 10.5 [plus or minus] 1.5 days; and albumin turnover rate, 6.7% [plus or minus] 0.93%/day, 17.2 [plus or minus] 2.7 g./day, 15.4 [plus or minus] 2.0 g./1.73 sq. m./day. In 6 normal subjects approximations of the intra- and extravascular albumin of the body indicated these quantities were of the same order of magnitude.