Merck
  • Home
  • Search Results
  • Plasma amino acid levels with a note on membrane transport: characteristics, regulation, and metabolic significance.

Plasma amino acid levels with a note on membrane transport: characteristics, regulation, and metabolic significance.

Nutrition (Burbank, Los Angeles County, Calif.) (2002-09-26)
Luc A Cynober
ABSTRACT

The plasma concentration of an amino acid (AA) is the result of its rates of appearance (Ra) in and disappearance (Rd) from plasma. As for most nutrients, AA Ra and Rd are tightly regulated and at the postabsorptive state Ra equals Rd. Factors controlling Ra are protein intake and tissue release; those controlling Rd are tissue uptake and body losses (urine, sweat, etc.). Regulation of plasma AA concentrations involves hormones, in particular insulin and glucagon, both of which induce hypoaminoacidemia (but for quite different reasons), and cortisol, which induces hyperaminoacidemia. In addition, in pathologic states, catecholamines, thyroid hormones, and cytokines modulate plasma AA levels. Peripheral availability of AAs after protein ingestion is controlled by the liver, with an activation of ureagenesis in hyperprotein feeding and repression during a hypoprotein diet. The arginine-to-citrulline pathway in the intestine plays a key role in this adaptative process. In some circumstances tissue uptake of AAs and further metabolism depend on plasma AA concentrations. Plasma glutamine level may be the driving force controlling the flux of this AA at the muscle level. Also, channeling of the arginine cellular pathways means that plasma arginine is a major controlling component of nitric oxide synthesis in endothelial and immune cells. All these features explain the excessive increase in glutamine and arginine demands, in particular for energy expenditure, leading to morbidity (e.g., gut atrophy, muscle wasting, and immune dysfunction) in stressed patients. Normoaminoacidemia is not synonymous with health because this state is observed in level 2 starvation (Ra and Rd decrease) or after minor injury (Ra and Rd increase). Hyperaminoacidemia may be the consequence of organ failure (Rd decreases) or excessive AA intake during parenteral nutrition (Ra increases). Hypoaminoacidemia is observed after organ removal (Ra decreases, e.g., decrease in citrulline concentration in short bowel syndrome) or in stress situations (Rd increases). Mere determinations of plasma AA concentrations at the basal state (i.e., postabsorptive) provide rather limited information. Their usefulness can be improved by measuring arteriovenous differences or performing time course measurements, but techniques based on stable isotopes are necessary to obtain more precise information on the behavior of a particular AA or group of AAs.

MATERIALS
Product Number
Brand
Product Description

Supelco
L-Phenylalanine, certified reference material, TraceCERT®
Sigma-Aldrich
1,2-Propanediol, SAJ special grade, ≥99.0%
Sigma-Aldrich
L-Phenylalanine, SAJ special grade, ≥99.0%
Sigma-Aldrich
Urea, JIS special grade, ≥99.0%
Sigma-Aldrich
Urea, ≥99.0%
Sigma-Aldrich
Urea, SAJ first grade, ≥98.0%
Sigma-Aldrich
1,2-Propanediol, SAJ first grade, ≥99.0%
Sigma-Aldrich
L-Isoleucine, SAJ special grade, ≥99.0%
Sigma-Aldrich
Tyramine, 98%, FG
Supelco
Taurine, Pharmaceutical Secondary Standard; Certified Reference Material
Sigma-Aldrich
L-Glutamic acid, JIS special grade, ≥99.0%
Sigma-Aldrich
1,2-Propanediol, ACS reagent, ≥99.5%
Sigma-Aldrich
1,2-Propanediol, ReagentPlus®, 99%
Sigma-Aldrich
L-Phenylalanine, 99%, FCC
Sigma-Aldrich
Urea, puriss. p.a., ACS reagent, reag. Ph. Eur., ≥99%
Sigma-Aldrich
L-Alanine-12C3, 99.9 atom % 12C
Sigma-Aldrich
Tyramine, ≥98.0%
Sigma-Aldrich
L-Isoleucine, 99%, FCC, FG
Sigma-Aldrich
1,2-Propanediol, meets analytical specification of Ph. Eur., BP, USP, ≥99.5%
Sigma-Aldrich
Urea, puriss., meets analytical specification of Ph. Eur., BP, USP, 99.0-100.5%, 99.0-101.0% (calc. on dry substance)
Sigma-Aldrich
L-Phenylalanine, BioUltra, ≥99.0% (NT)
Sigma-Aldrich
L-Glutamic acid, BioUltra, ≥99.5% (NT)
Sigma-Aldrich
1,2-Propanediol, puriss. p.a., ACS reagent, ≥99.5% (GC)
Sigma-Aldrich
L-Isoleucine, BioUltra, ≥99.5% (NT)
Sigma-Aldrich
L-Serine, BioUltra, ≥99.5% (NT)
Sigma-Aldrich
1,2-Propanediol, tested according to Ph. Eur.
Supelco
L-Isoleucine, certified reference material, TraceCERT®
Supelco
L-Asparagine, certified reference material, TraceCERT®
Supelco
L-Glutamic acid, certified reference material, TraceCERT®
Supelco
L-Glutamine, Pharmaceutical Secondary Standard; Certified Reference Material