Amino acid profiling clinical guidelines for determination of preferred specimen choice

Townsend Letter for Doctors and Patients,  Dec, 2003  by Bradley Bongiovanni,  Judy Feinerman

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Introduction

Profiling of amino acids in plasma and urine has been used to elucidate a rapidly growing number of aminoacidopathies since the introduction of partition chromatography methods in 1945. (1) The question of whether plasma or urine may be the preferred specimen choice for amino acid testing is a frequent clinical concern in the evaluation of a patient's amino acid status. An informed decision must involve what principal clinical answers are sought and which amino acids are being tested. To state categorically that profiling of amino acids is best performed on plasma or urine is to oversimplify. The question of preferred specimen can be answered only when it is addressed to specific amino acids or to the specific type of information desired.

One commonly practiced method to judge the relative value of results from two specimen types is to ask which specimen has been most used for scientific studies. The majority of published studies have used plasma as the specimen for analysis (approximately a 3:1 plasma/urine ratio). (2) This is primarily because most investigations have been concerned with essential amino acid status. Urine is typically reserved for studies of dietary protein intake, digestive adequacy, bone loss and muscle protein catabolic states. The aminoacidemias and aminoacidurias associated with metabolic disorders are approximately equally divided in the published research. Inherited metabolic disorders generally result in extreme elevations, and the abnormality is easily detected in either specimen type. The branched chain amino acids (BCAAs), for example, are elevated in both plasma and urine in maple syrup urine disease. The newer application of amino acid profiling of older children and adults to determine amino acid status in chronic degenerative diseases is more pertinent for this article.

Amino Acid Dynamics

Plasma

A fasting plasma specimen reflects the state of the dynamic flux of amino acids leaving sites like skeletal muscle and flowing into sites of utilization in liver, brain, and other tissues (Figure 1). Amino acid levels in plasma reach their homeostatic balance point making a fasting specimen ideal for repeated measures to monitor progress. The principal factors effecting changes over time are dietary intake, digestive efficiency, hepatic uptake, and the ability of skeletal muscle to maintain sufficient rates of transamination. The amount of an essential amino acid in plasma determines the rate of any dependent process in the tissues. For example, low plasma tryptophan results in reduced formation of serotonin in the brain. (3)

[FIGURE 1 OMITTED]

Urine

Twenty-four hour urinary amino acids have been measured in the evaluation of specific clinical conditions. In many cases the research represents disruption of normal amino acid metabolism as a result of the disease process and the shortterm changes in plasma amino concentration. A 24 hour urine amino acid analysis reveals amino acid metabolism throughout the period of metabolic stress of digestion and daily activity. This aspect is of particular value for evaluating those amino acids that primarily reveal tissue degradation, such as hydroxylysine and hydroxyproline, which are released from collagen of connective tissue and bone.

Clinical Categories Assessed via Amino Acid Profiling

Gastrointestinal Function

Amino acids and their derivatives provide some useful markers that can reflect gastrointestinal function, specifically protein digestion capacity. The normal digestion of dietary protein results in free-form amino acids and short-chain peptides. Recent (i.e. 3 days) dietary protein intake has little influence on plasma amino acid profiling. A fasting plasma specimen highlights the dynamics of homeostatic maintenance of the free form amino acid pool, which is remarkably stable, independent of diet. In contrast, 24-hour urine analysis of amino acids more clearly elucidates recent protein intake based on the activities of the previous 24-48 hours. In feeding young men a protein mixture (patterned after egg protein) specifically devoid of methionine and cystine for eight days, fasting plasma methionine and cystine levels showed little change during the eight-day period. Urinary levels of methionine decreased markedly within a few days after feeding of the experimental diet, suggesting urinary amino acids are more useful to monitor short-term changes in protein intake. However, plasma levels are the preferred way to assess long-term adequacy and dynamics of amino acid utilization. (4), (5)

Abnormal amino acid patterns can correspond to what may be wrong in protein nourishment or digestion. The patterns seen may reflect dietary protein deficiency, and/or maldigestion. Hyperaminoacidemias and hyperaminoacidurias typically indicate genetically inherited metabolic enzyme impairments or transport problems, not digestive enzyme impairments or insufficient stomach acid secretion. Low levels measured among the essential and some of the semi-essential amino acids reflect dietary and uptake problems. For example, the essential amino acid histidine is required to make histamine, an important digestive function, which occurs early in the stomach. Low plasma or urinary histidine may then suggest impaired ability for optimal protein digestion. Low levels of the aromatic amino acids--tryptophan, phenylalanine, and tyrosine--may indicate inadequate stomach acid (HCl) secretion as this is critical to activate pepsin-mediated protein digestion. Clinicians must remember to consider renal function in evaluation of urinary amino acids, however, as patients with renal failure may show decreased creatinine measurements, resulting in skewed levels upon measurement and subsequent interpretation.