N-Acetyl-L-Tyrosine as a Tyrosine Source in Adult Parenteral Nutrition
Hoffer, L John
Safe and effective delivery of parenteral nutrition requires the infusion of crystalline amino acids in physiologic amounts and proportions.1 Tyrosine’s limited water solubility poses a problem in this regard, and for this reason, the 10% formulation of the widely used product, Aminosyn (Abbott Laboratories, SaintLaurent, QC) contains only 44 mg tyrosine/100 mL. Tyrosine is synthesized from phenylalanine in the liver and kidney,2 so when phenylalanine hydroxylase activity is normal, a sufficiently large amount of phenylalanine will compensate for absent tyrosine in a nutrient admixture. Recently published recommended dietary allowances (RDA) for phenylalanine and tyrosine are expressed as their sum: 36 mg/kg-day^sup -1^.3 Aminosyn 10% provides 440 mg phenylalanine/100 mL, so a patient infused 1.0 g/kg-day^sup -1^ total amino acids from this product will receive 44 mg phenylalanine/kg-day^sup -1^ and 4 mg tyrosine/kg-day^sup -1^. This exceeds the combined aromatic amino acid RDA and an earlier estimate that the average normal phenylalanine requirement of adults, in the absence of tyrosine, is 39 mg/kg-day^sup -1^.4
The water solubility of N-acetyl-L-tyrosine (NAT) initially suggested it could be used as a substitute for tyrosine in amino acid mixtures, and this was justified by rat studies demonstrating its prompt deacylation to tyrosine and incorporation into proteins.5 The situation is more complicated in humans, however.6 Heird et al7,8 measured plasma amino acid concentrations, growth, nitrogen (N) balance, and urinary NAT excretion in parenterally nourished low-birth-weight and normal infants and children infused with an amino acid mixture that supplied 30% of tyrosine substrate as tyrosine and 70% as NAT (TrophAmine; Kendall McGaw Laboratories, Irvine, CA). N balance was positive despite subnormal plasma tyrosine levels and the recovery in urine of approximately 25% of the infused NAT.7,8 Studying the same product in clinically stable infants, Christensen et al9 reported that one-third of the infused NAT was cleared by the kidneys. Van Goudoever et al10 reported that 38% of the NAT in a variety of NAT-containing amino acid solutions was excreted in the urine of low-body-weight premature infants.
Druml et al11-13 found approximately 60% of an IV dose of NAT was excreted unchanged in the urine of healthy adults, with no rise in plasma tyrosine concentrations either in healthy adults or patients with hepatic or renal disease. Magnusson et al14 likewise reported that 56% of a large IV dose of NAT, infused over 4 hours, was excreted unchanged in the urine. Gazzaniga et al15 administered TrophAmine to adult patients by continuous infusion, and reported that no more than 30% of the sum of infused tyrosine plus NAT was excreted in the urine as the sum of tyrosine and NAT.
More than 10 years ago, Abbott introduced a new product, Aminosyn II, in which NAT entirely replaced tyrosine. The 10% version of this product supplies 270 mg NAT/100 mL but only 298 mg phenylalanine/100 mL. A patient infused 1.0 g/kg-day^sup -1^ total amino acids as Aminosyn II will receive 30 mg phenylalanine/kg-day^sup -1^ and 22 mg NAT/kg-day^sup -1^, a total that exceeds their combined RDA.3
Because NAT is slowly deacylated and inefficiently reabsorbed by the renal tubules, situations could arise in which the total aromatic amino acid requirement of patients receiving this product is not met.6 However, apart from the study by Gazzaniga et al,15 who tested a pEdiatriC formulation that included both NAT and tyrosine, NAT retention has not been measured in adult patients receiving it in the customary way, as a constant infusion. To obtain information about this important nutritional issue we measured NAT retention of adult patients receiving Aminosyn II by continuous infusion.
MATERIALS AND METHODS
Study Design
Over a 2-month period, 13 patients receiving parenteral nutrition with Aminosyn II 15% were asked to provide 24-hour urine collections. Patients were approached to participate in this observational study if their parenteral nutritional composition and infusion rate were constant for 3 days, they were continent of urine or had their urine collected with an indwelling urinary catheter, and they were not receiving blood products. The study protocol was approved by the Research Ethics Committee of the Jewish General Hospital. All patients approached agreed to participate.
Methods
Urinary urea and creatinine were measured in the clinical chemistry laboratory using a Hitachi 917 automated analyzer (Laval, QC). Urinary NAT was determined by enzymatically converting NAT to tyrosine, measuring the resulting total tyrosine, and correcting the result for the small amount of native tyrosine in a separate, untreated aliquot. The method is similar to that described by Ogwu and Cohen16 for converting N-acetylcysteine to cysteine. Except for the amine derivatizing reagent, phenylisothiocyanate (PITC), which was from Pierce Chemical Company (Rockford, IL) all standards and reagents were from Sigma-Aldrich Canada, Ltd (Oakville, ON). Stock standards of tyrosine (3 mmol/L) and NAT (25 mmol/L) were made up in 50 mmol/L HCl. To 0.15 mL of 50 mmol/L sodium phosphate buffer in 0.2 mmol/L ethylenediamine tetraacetic acid (EDTA) (pH 7.0) were added 0.075 mL standard or well-mixed undiluted urine sample followed by 0.025 mL porcine kidney Acylase I (Sigma no. A-3010) made up just before use by mixing it in the phosphate-EDTA buffer to a concentration of 60 mg/mL. The enzyme substrate mixture was promptly incubated for 30 minutes at 37°C in a shaking water bath. Longer incubation periods did not increase the yield. The reaction was terminated by adding 1.25 mL ice-cold methanol, followed by mixing and centrifugation for 15 minutes at 4°C to remove precipitated proteins. Supernatant (0.4 mL) was promptly removed and dried under a stream of nitrogen gas at 70°C, then converted to the PITC derivative for separation and analysis by reverse-phase high-performance liquid chromatography with ultraviolet detection as previously described.17 The tyrosine peak eluted at 9.2 minutes. Each sample run included tyrosine standards and an NAT standard. Recovery of NAT added to urine samples was 100%. NAT assayed in the absence of enzyme gave no signal, and urine specimens from 3 patients receiving Aminosyn (which contains no NAT) showed an absence of NAT. NAT excretion was calculated as NAT excretion minus tyrosine excretion measured separately in a different sample of the urine collection.
Calculations
All parenteral nutrients were admixed by using the computer-controlled Baxa Corporation MicroMacro 12 Compounder (Englewood, CO), which accurately records final solution concentrations. The infusion rates of amino N and NAT were calculated as the product of their concentration in the final infusate delivered to the patient and the constant infusion pump rate. Because all nutrients were infused at a constant rate for at least 3 days before and throughout the 24-hour collection period, the input rate per 24 hours was known. The timing of 24-hour urine collections was optimized by carefully following each patient and repeatedly explaining the study to the nurses. To detect urine collections that might inadvertently be too short or too long, more than 1 collection was obtained from several patients and urinary creatinine excretion was scrutinized. N balance (g/day) was calculated as N^sub in^ – urea nitrogen^sub out^ – 4 g to account for nonurea N and nonrenal losses. Percentage NAT excretion was calculated as 100 × NAT^sub out^/NAT^sub in^. Data are presented as the means ± SD.
RESULTS
Table I shows the characteristics of the 13 patients, who provided as few as 1 or as many as 4 urine collections. Repeat urine collections were separated by an average of 8 ± 4 days; the shortest intercollection interval was 2 days. Capillary blood glucose was routinely monitored and was almost always
DISCUSSION
This study indicates that when a product in which all tyrosine is supplied as NAT is provided by continuous infusion, approximately 35% of the NAT infused is excreted unchanged in the urine. This is less than the approximately 60% loss previously reported in adult studies in which NAT was infused rapidly11-14 and somewhat greater than the maximum 30% loss reported in an earlier study of adults infused with a pediatric formulation that supplied both NAT and tyrosine.15 Few details of the urinary NAT measurements are provided in that study,1 which grouped both intake and excretion as the sum of tyrosine and NAT. Because nearly all of the sum of urinary NAT plus tyrosine must have been NAT, whereas 70% of the sum of infused NAT plus tyrosine was NAT, specific NAT excretion was probably approximately 40%, similar to the present findings.
The present study was carried out with the narrow aim of determining what fraction of continuously infused NAT is retained by adult patients receiving Aminosyn II by constant infusion and not the more difficult one of proving whether or not its aromatic amino acid content is compatible with optimum N balance. However, our results provide no grounds for suspecting inadequate aromatic amino acid provision. If one conservatively assumes that 50% of infused NAT is lost in the urine, a typical patient prescribed 1.0 g/kg-day^sup -1^ total amino acids from Aminosyn II would retain 30 mg phenylalanine/kg-day^sup -1^ and 11 mg NAT/kg-day^sup -1^. The total of 41 mg/kg-day^sup -1^ aromatic amino acids retained is greater than the current combined RDA of 36 mg/kg-day^sup -1^.3 The amounts of phenylalanine and NAT supplied by this product appear such that adults receiving it in a dose of 1.0 g total amino acids/kg-day^sup -1^ will meet their combined aromatic amino acid requirement, even though a substantial fraction of the NAT is excreted unchanged in their urine.
In spite of these reassuring results, clinicians who infuse IV amino acid solutions reliant on NAT should remain alert for situations in which phenylalanine conversion to tyrosine (or potentially of NAT to tyrosine) is impaired, as may occur in renal or hepatic disease.2,12,13,20,21 Especially important might be the coexistence of conditions that further reduce the already poor efficiency of renal tubular NAT.reabsorption, such as very rapid amino acid infusions (as in cyclic parenteral nutrition), increased renal flow,22 and, possibly, severe trauma.23 This study does not address the tyrosine needs of parenterally fed neonates. Roberts et al24 recently estimated the tyrosine requirement of human neonates to be 94 mg/kg-day^sup -1^, considerably higher than in currently available amino acid solutions.
ACKNOWLEDGMENTS
The study was supported by Canadian Institutes of Health Research Grant MOP8725 (LJH) and a summer medical student bursary to KS from the Faculty of Medicine, McGiIl University. We thank the nursing staff involved and staff of the Department of Diagnostic Medicine for their cooperation with the urine collections.
Copyright American Society for Parenteral and Enteral Nutrition Nov/Dec 2003
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