Direct measurement of the rates of synthesis of plasma proteins in control subjects and patients with gastrointestinal protein loss

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Abstract
The guanido carbon of hepatic arginine is the common precursor of urea and of the arginine of plasma proteins synthesized in the liver. It is possible to measure the momentary synthetic rates of plasma proteins by “pulse labeling” this arginine pool with bicarbonate-14C. In the current study, this method has been adapted in order to use urinary urea data and was applied to control subjects and patients with gastrointestinal protein loss. The assumptions required for this determination are discussed. There was close agreement between albumin synthetic rates measured by this method and albumin catabolic rates derived from simultaneous albumin-131I studies, supporting the validity of the method and suggesting that there is relatively little fluctuation in the rate of albumin synthesis from time to time. The albumin synthetic rates in six control subjects averaged 5.8 mg/kg per hr, while those of five patients with gastrointestinal protein loss averaged 7.2 mg/kg per hr. Thus in these patients, there was relatively little acceleration of albumin synthesis in response to continued loss of plasma proteins into the gastrointestinal tract. Fibrinogen synthetic rates averaged 1.9 mg/kg per hr in five control subjects and 3.2 mg/kg per hr in five patients with gastrointestinal protein loss. Transferrin synthetic rates exhibited considerable individual variation in both groups and averaged 0.24 mg/kg per hr in four control subjects and 0.31 mg/kg per hr in five patients with gastrointestinal protein loss. The method employed in this study offers several advantages in studying plasma protein metabolism. It provides a direct measurement of protein synthesis, applicable to several proteins simultaneously, does not require a long-term steady state in the metabolism of the proteins, and is capable of measuring short-term fluctuations in synthetic rates. Therefore, this approach is applicable to the investigation of the physiological factors controlling the rates of synthesis for plasma proteins.