Plasma Protein Turnover and Tissue Exchange

Abstract

Chemical nitrogen balance studies leave much to be desired in supplying information about internal protein metabolism, relying, as they do, on net differences between intake and output. By isotopically labeling plasma and tissue proteins valuable information concerning the turnover, transport, and equilibria of these proteins under normal basic conditions has been obtained. The present studies deal with some of these aspects of internal metabolism under conditions of altered nitrogen balance.The protein metabolic status of a group of normal dogs was modified by varying the protein content of the diet from 0 to 6 gm. of protein per kilogram of body weight per day. Carbon-14-1abeled homologous plasma proteins were then injected intravenously. The daily plasma protein turnover rate in these animals was accelerated from 0.65 to 1.00 gm. per kilogram by increasing the oral protein intake from 0 to 2.0 gin. Further increases were not observed with oral protein increments up to 6.0 gin. per kilogram of body weight. Turnover of both albumin and globulins was increased and was associated with a greater exchange of C I~ between plasma and tissue proteins and a greater excretion of C 14 in the expired air.In protein-depleted animals receiving a very low protein diet the rate of turnover as indicated by a shorter half-life, was also increased. However, owing to a marked reduction in the total plasma protein pool the amount metabolized per day was reduced.In a small group of animals with both plasma and tissue proteins simultaneously labeled by feeding DL-lysine-6-C ~4 a significant recycling of C 14 from

The problem of nitrogen disposal in the obese

“…The latter may be related to the higher levels of circulating lipopolysaccharide observed in the metabolic syndrome (123) , also linked to the maintenance of low-key inflammation (124,125) caused by increased intestinal bacteria activity (126) . These findings hint to the postulated excess 2-amino-N, in agreement with the higher availability of amino acids and energy to increase protein turnover (127,128) and to maintain a fully functional immune system (129) observed in the metabolic syndrome.…”

Dietary protein regulates hepatic constitutive protein anabolism in rats in a dose-dependent manner and independently of energy nutrient composition

“…This was in line with previous results obtained in rats and dogs (14,42) and was also consistent with data in humans showing the lack of effect of diets supplying 9% or 24% of energy as protein, but containing identical levels of energy and fat, on albumin, fibrinogen, or fibronectin synthesis rates (6). These results also agree with the weak association demonstrated between albumin FSR and the level of dietary protein, when these ranged from the recommended daily allowance to an excess value of 25% (39).…”

Micropuncture studies of proximal tubule albumin concentrations in normal and nephrotic rats

“…The concentration of albumin in the glomerular filtrate of a number of animal species has been estimated to be in the order of 20 mg/100 ml (1-4), a small fraction being Received for publication 26 October 1970 and in revised form 18 January 1971. excreted while the remainder is absorbed by the renal tubule epithelial cells. If this were true of the rat, for instance, the animal would filter and absorb an amount of albumin equivalent to its total albumin pool each day (5). Renal micropuncture studies in rats and dogs have revealed albumin concentrations in proximal tubule fluid in the order of 2-10 mg/100 ml or more (6)(7)(8)(9)(10).…”