Measurement of plasma protein and whole body protein metabolism using [¹⁵N]glycine in a young adult man – a pilot study

A novel simplified method is presented for the estimation of the metabolism of plasma proteins (albumin, fibrinogen, <i>α</i>, <i>β</i> and <i>γ</i>-globulin, glycoprotein) with regard to the whole body protein metabolism in a young male volunteer (22 years, 81 kg body mass). This method is based on multiple oral administration of [¹⁵N]glycine followed by measurement of ¹⁵N in plasma proteins, total free amino acids, urea and excreted urinary N. The fractional synthesis rate of albumin was estimated to 6.8 % d⁻¹ based on amino acids and 3.3 % d⁻¹ based on urea, respectively. The fractional synthesis rate of the other plasma proteins ranged from 4.3 % d⁻¹ (<i>γ</i>-globulin) to 26.4 % d⁻¹ (<i>α</i>-globulin, fibrinogen). We conclude that the simplified approach using [¹⁵N]glycine provides results which are similar to results based on the simultaneously applied ¹³¹I-human serum albumin technique as ‘gold standard’ and to those reported in literature. The compartmental analysis considering comprehensive tracer kinetic data ensures reliable data treatment and enables statistical evaluation. The analytical effort is minimal because the ¹⁵N enrichment of plasma protein after chemical digestion may be directly used. Therefore, the novel stable isotope ¹⁵N method is suitable for studies in clinical and nutritional research and practice.