The arginine-6-14C (14C-bicarbonate) method of measuring plasma protein synthetic rate and its simplification
Figure 8.1 introduces the subject of this paper which is how to simplify and make more generally usable the 14C-bicarbonate method of measuring synthetic rate of plasma proteins synthesised by the liver. It shows measurements of the specific radioactivity of the arginine 6-carbon of plasma albumin and plasma fibrinogen obtained about 3 hours after one hour’s intravenous infusion of 0.5 mC 14C-bicarbonate. Rabbits were injected with 5 mg of cycloheximide per kg of body weight from 1 to 48 hours before the start of the infusion and the specific radioactivities are expressed as fractions of the mean levels we found in untreated healthy control animals. It is seen that the specific radioactivities (SA) of both albumin and fibrinogen arginine-6-C, are reduced almost to zero shortly after the injection, but both show some recovery 6 hours later; albumin SA remains unchanged 12 hours later but by this time fibrinogen arginine-6-C SA has rebounded to more than twice its initial levels; by 24 hours albumin arginine-6-C SA has returned to initial levels while fibrinogen arginine-6-C has fallen to 1.5 times initial levels; and by 48 hours both albumin arginine-6-C and fibrinogen arginine-6-C SA are near initial levels. This is a rather remarkable contrast in the responses of two different liver-synthesised plasma proteins to an agent that blocks ribosomal translation of mRNA1,2. We should like to be able to interpret such results in terms of the underlying albumin and fibrinogen synthesis.
KeywordsUrea Cycle Specific Radioactivity Plasma Fibrinogen Synthetic Rate Carbonate Carbon
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