Infants protein metabolism

Des Robert C, Le Bacquer O, Piloquet H, et al. Acute effects of intravenous glutamine supplementahon on protein metabolism in very low birth weight infants a stable isotope study. Pediatr Res 2002 51 87-93. 

Although there is limited experience in managing pregnancies in these disorders, it has become apparent that the risk for metabolic decompensation increases for the mother, especially during the postpartum period when protein catabolism is greatest. However, unlike in PKU, it appears that the infant may not be at increased risk of adverse outcomes in these other protein metabolism disorders. This chapter will review what has been learned about managing pregnancies in these intoxication disorders. 

Protein metabolism was estimated in 3 premature infants between 40 and 70 days after birth [412]. The rate of protein synthesis and the metabolic pool size were greatly increased when compared with normal adult values. 

The level of blood urea nitrogen (BUN), a by-product of protein metabolism, is used to assess renal function and has an adult normal range of 10-20 mg/dL (3.6-7.1 mmol/L). The range for an infant or child is 5-18 mg/dL for and slightly lower for the newborn. 

Bresson JL, Bader B, Rocchiccioli F, et al. Protein-metabolism kinetics and energy-substrate utilization in infants fed parenteral solutions with different glucose-fat ratios. Am J Clin Nutr... 

PHOSPHOPROTEINS. These proteins have phosphate groups esterified to the hydroxyls of serine, threonine, or tyrosine residues. Casein, the major protein of milk, contains many phosphates and serves to bring essential phosphorus to the growing infant. Many key steps in metabolism are regulated between states of activity or inactivity, depending on the presence or absence of phosphate groups on proteins, as we shall see in Chapter 15. Glycogen phospho-rylase a is one well-studied example. 

The urea cycle is essential for the detoxification of ammonia 678 Urea cycle defects cause a variety of clinical syndromes, including a metabolic crisis in the newborn infant 679 Urea cycle defects sometimes result from the congenital absence of a transporter for an enzyme or amino acid involved in the urea cycle 680 Successful management of urea cycle defects involves a low-protein diet to minimize ammonia production as well as medications that enable the excretion of ammonia nitrogen in forms other than urea 680... 

Amino acid analysis has long been of importance for nutritional purposes. Recently, there has been an increasing interest in assessing the exact amino acid composition of new protein sources, infant formulas, nutraceutical-type products, or supplements targeted at people with unusual metabolic needs. There is also a regulatory necessity to have rapid methods available for amino acid determination to verify product consistency with that declared on the label [196]. 

The absorption of vitamins K2, which are found mainly in cheese, curd cheese, and natto, is much higher and may be almost complete. Thus the nutritional importance of menaquinones is often underestimated. The vitamin K activity is related to the activation of specific proteins involved in blood clotting and bone metabolism. Clinical vitamin deficiency due to dietary inadequacy is rare or nonexistent in healthy adults, thanks to the widespread distribution of the vitamin K in foodstuffs and the microbiological flora of the gut, which synthesizes menaquinones. Only infants up to 6 months are at risk of bleeding due to a vitamin K deficiency. No data on negative effects of an overdose of vitamin K are found [417]. 

Pharmacokinetics Rapidly absorbed from the G1 tract. Protein binding 92%-94%. Widely distributed. Metabolized in the liver to some active metabolites. Primarily excreted in urine. Not removed by hemodialysis. Half-life 2.4-3 hr (increased in impaired renal function and premature infants). 

Mechanism of Action Asynfheficisomer of thyroxine involved in normal metabolism, growth, and development, especially of fhe CNS in infants. Possesses catabolic and anabolic effects. Therapeutic Effect Increases basal metabolic rate, enhances gluco-neogenesis and stimulates protein synthesis. 

Lung maturation in the fetus is regulated by the fetal secretion of cortisol. Treatment of the mother with large doses of glucocorticoid reduces the incidence of respiratory distress syndrome in infants delivered prematurely. When delivery is anticipated before 34 weeks of gestation, intramuscular betamethasone, 12 mg, followed by an additional dose of 12 mg 18-24 hours later, is commonly used. Betamethasone is chosen because maternal protein binding and placental metabolism of this corticosteroid is less than that of cortisol, allowing increased transfer across the placenta to the fetus. 

Today, many of the older methods used historically in clinical chemistry are being replaced by methods that utilize MS. In 2013, some areas of interest in clinical chemistry include steroid and vitamin D analyses that require a high selectivity and relative sensitivity. For example, the improved selectivity is essential in the analysis of testosterone in infants due to low concentrations. Immunoassays are not sufficiently selective to provide an accurate measurement of the true concentrations of these metabolites. There are many more assays being used or that are now in development in clinical chemistry, but a discussion of this growing use of metabolic screening is too broad and too evolutionary to be included here. Therefore, this chapter focuses on those proteins and metabolites (e.g., amino acids and acylcamitines) that are part of a classic newborn screening panel. 

To study the plasma protein binding at least in newborns and infants is recommended, as the protein binding is reduced in the preterm and term infant at birth and in the first weeks of life. Also, the drug elimination is slowed down in this subgroup of children, due to immaturity of both metabolic pathways and renal function. 

---