Isotopic and modeling investigation of long-term protein turnover in rat tissues

Abstract

Fractional synthesis rates (FSR) of tissue proteins (P) are usually measured using labeled amino acid (AA) tracer methods over short periods of time under acute, particular conditions. By combining the long-term and non-steady-state (15)N labeling of AA and P tissue fractions with compartmental modeling, we have developed a new isotopic approach to investigate the degree of compartmentation of P turnover in tissues and to estimate long-term FSR values under sustained and averaged nutritional and physiological conditions. We measured the rise-to-plateau kinetics of nitrogen isotopic enrichments (δ(15)N) in the AA and P fractions of various tissues in rats for 2 mo following a slight increase in diet δ(15)N. Using these δ(15)N kinetics and a numerical method based on a two-compartment model, we determined reliable FSR estimates for tissues in which P turnover is adequately represented by such a simple precursor-product model. This was the case for kidney, liver, plasma, and muscle, where FSR estimates were 103, 101, 58, and 11%/day, respectively. Conversely, we identified tissues, namely, skin and small intestine, where P turnover proved to be too complex to be represented by a simple two-compartment model, evidencing the higher level of subcompartmentation of the P and/or AA metabolism in these tissues. The present results support the value of this new approach in gaining cognitive and practical insights into tissue P turnover and propose new and integrated FSR values over all individual precursor AA and all diurnal variations in P kinetics.

Impact on bulk 15N natural isotopic abundance in hair of kidney function in type 2 diabetic nephropathy

“…We showed in the latter study that by examining compound specific amino acid 15 N NIA values in the nutrient source (mother's milk) and infant protein (hair at one month), the bulk 15 N NIA of the infant appears to be dominated by infant amino acid metabolism, not by maternally-derived nutrients. This conclusion is coherent with that of Poupin et al [4,27]. for a range of tissues in rat, indicating that such data might be exploited in the same way to examine modified protein turnover in relation to pathological state in humans.…”

“…As hair keratin is continuously accumulated, thus excluded from the pool of amino acids, its 15 N NIA values therefore reflect the nitrogen metabolic status at the time of 15 N incorporation into newly synthesized proteins. Hence, the value at any given time-point in hair growth will retain a measure of fluctuations in both the protein fractional synthesis rate and in the intracellular free amino acid precursor pool [4,27]. We previously used this method to examine nutrition in newborns [18] and in fully breastfed infants [19,28].…”

“…The rate of protein synthesis is enhanced in obesity [1] and in type 1 and type 2 diabetes mellitus [2,3]. Accessing modifications to protein turnover is not facile and is classically carried out using short-term isotopictracer techniques [4]. Such an approach is restricted to short-term observations, however, and other methods are needed by which longer-term measurements can be obtained.…”

“…NIA of whole body proteins partly depends on protein source, being higher in animal-than vegetable-derived proteins [5]. In addition, recent studies in rats exploiting both low-enriched 15 N-amino acid perfusion [4] and dietary-related studies [6] suggest that protein metabolism is a major determinant of bulk 15 N NIA values, as much as is protein intake. Amongst the factors contributing to the metabolic component determining bulk 15 N NIA is the transamination rate [7] and urea cycle flux rate [8].…”

Do we excrete what we eat? Analysis of stable nitrogen isotope ratios of human urinary urea

“…Both blood and urinary urea concentrations have been used for clinical investigations of protein metabolism and renal malfunction. In mammals, natural stable isotope analyses of urea have been employed for metabolic studies . Previous studies have assessed the effects of nutritional restriction on urinary urea δ 15 N values in reindeer, caribou and muskoxen, demonstrating that in mammals the δ 15 N values of urinary urea are reflective of the nutritional state.…”