Protein, amino acid turnover hemoglobin

This metabolic activity is achieved by a turnover of amino acids and proteins that is as rapid as that of lipids and carbohydrates. In an adult human male, 400 g of body proteins is turned over each day. Of this, 50g is used to replace digestive enzymes (Sec. 15.2), and 6g to replace hemoglobin (Sec. 15.8). The concentration of free amino acids in plasma is small (total 3.2mmol L l, of which 25 percent is glutamine), but the turnover of 400g per day of protein is equivalent to the uptake, and release back into the plasma, of 4.6 moles of a-amino-N, so that the average lifetime of an amino... 

The dynamic process in which body proteins are continuously hydrolyzed and resynthesized is called protein turnover. The turnover rate, or life expectancy, of body proteins is a measure of how fast the proteins are broken down and resynthesized. The turnover rate is usually expressed as a half-life. The use of radioactive amino acids has enabled researchers to estimate half-lives by measuring the exchange rate between body proteins and the amino acid pool. For example, the half-life of liver proteins is about 10 days. This means that over a 10-day period, half the proteins in the liver are hydrolyzed to amino acids and replaced by equivalent proteins. Plasma proteins also have a half-life of about 10 days, hemoglobin about 120 days, and muscle protein about 180 days. The half-life of collagen, a protein of connective tissue, is considerably longer—some estimates are as high as 1000 days. Other proteins, particularly enzyme and polypeptide hormones, have much shorter half-lives of only a few minutes. Once it is released from the pancreas, insulin has a half-life estimated to be only 7-10 minutes. 

Progress has been made in demonstrating synthesis of specific proteins by cell-free systems, e.g., on the synthesis of cytochrome c by isolated rat liver mitochondria (S3, cf. 383) and on the synthesis of serum albumin by the isolated microsome fraction of rat liver 34, cf. 335,336). Campbell et al. 34) concluded that while specific serum albumin is, indeed, synthesized on the ribonucleoprotein particle fraction of the microsomes, it is not readily released in soluble form. In other words, the isolated microsomes have lost their ability to promote substrate turnover. The same is true for the hemoglobin-synthesizing RNP particles from rabbit reticulocytes 35, cf. 138). Ogata and associates 36) have essentially confirmed the results of Campbell et al. 34) on the synthesis of serum albumin by liver microsomes they have also studied the relative effect of both stimulatory and inhibitory factors on the incorporation of different amino acids into total ribonucleoprotein and into serum albumin, and showed that the requirements for the two processes were generally the same it may be noted, however, that pretreatment of the pH 5 enzymes with ribonuclease, which caused a 95% inhibition of the ineorporation into ribonucleoprotein, inhibited the corresponding incorporation into serum albumin by only 55%. 

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