RBP Turnover

For cynomolgus monkeys (2.8-4.0 kg body weight), the biological half-times observed for RBP and TTR (using RBP with normal affinity for TTR) were 6.6 h and 22-23 h, respectively. Free RBP, without affinity for TTR, showed a biological half-time of only 1.9 h. 

These data indicate that the turnover of RBP in vivo is quite rapid, with a fairly high body synthetic (production) rate for a protein of such low plasma concentration. This rapid turnover rate underlies the potential usefulness of RBP measurements in helping to assess the functional state of the liver in patients with hepatic disease, or the nutritional status of patients with borderline or actual malnutrition (see discussion below under clinical studies). It is of interest to compare the turnover of RBP with that of other plasma proteins. It has been pointed out (F. R. Smith et al., 1975) that the respective half-lives (in days) and synthetic rates (in milligrams per kilogram per day) in normal adult human subjects are approximately 0.5 and 5 for RBP 2-3 and 8-9 for TTR (Vahlquist et al., 1973 Socolow et al., 1965) and 14 and 200 for albumin (Beeken et al., 1962). 

The estimated total body turnover rate of RBP can be compared with available estimates for total body vitamin A utilization and turnover in normal well-nourished adults. A body turnover rate for RBP of about 5 mg kg day is equivalent to 16-17 pmol turnover per day for a 70-kg man. A comparable turnover for retinol would represent the daily turnover of 4.6-4.S mg, or approx- 

---