previously in a Northern American population (8, 11 ...

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previously in a Northern American population (8, 11 ), which may be explained by a higher intake of folate and B vitamins in the American population, as discussed by den Heyer et al. (12 ). In conclusion, tCys, which is an essential amino acid in the human fetus, may be actively transported in the placenta from the maternal to the fetal circulation where it is used in biosynthesis. Similarly, the fetus may extract tHcy from the maternal circulation.

This study was supported by Grant 28-2801.1 from the “Praeventiefonds”, The Hague, The Netherlands. We thank Claudia van Alphen for patient data management. References 1. Stamler JS, Slivka A. Biological chemistry of thiols in the vasculature and in vascular-related disease. Nutr Rev 1996;54:1–30. 2. Raijmakers MTM, Zusterzeel PLM, Steegers EAP, Hectors MPC, Demacker PNM, Peters WHM. Plasma thiol status in preeclampsia. Obstet Gynecol 2000;95:180 – 4. 3. Chappell LC, Seed PT, Briley AL, Kelly FJ, Lee R, Hunt BJ, et al. Effect of antioxidants on the occurrence of preeclampsia in women at increased risk: a randomised trial. Lancet 1999;345:810 – 6. 4. Davidge ST. Oxidative stress and altered endothelial cell function in preeclampsia. Semin Reprod Endocrinol 1998;16:65–73. 5. Chien PF, Smith K, Watt PW, Scrimgeour CM, Taylor DJ, Rennie MJ. Protein turnover in the human fetus studied at term using stable isotope tracer amino acids. Am J Physiol 1993;265:E31–5. 6. Ronzoni S, Marconi AM, Cetin I, Paolini CL, Teng C, Pardi G, et al. Umbilical amino acid uptake at increasing maternal amino acid concentrations: effect of a maternal amino acid infusate. Am J Obstet Gynecol 1999;181:477– 83. 7. Cetin I, Ronzoni S, Marconi AM, Perugino G, Corbetta C, Battaglia FC, et al. Maternal concentrations and fetal-maternal concentration differences of plasma amino acids in normal and intrauterine growth-restricted pregnancies. Am J Obstet Gynecol 1996;174:1575– 83. 8. Malinow MR, Rajkovic A, Duell PB, Hess DL, Upson BM. The relationship between maternal and neonatal umbilical cord plasma homocyst(e)ine suggests a potential role for maternal homocyst(e)ine in fetal metabolism. Am J Obstet Gynecol 1998;178:228 –33. 9. Kloosterman GJ. On intrauterine growth. The significance of prenatal care. Int J Gynaecol Obstet 1970;8:175–7. 10. Vina J, Vento M, Garcia Sala F, Puertes IR, Gasco E, Sastre J, et al. L-Cysteine and glutathione metabolism are impaired in premature infants due to cystathionase deficiency. Am J Clin Nutr 1995;61:1067–9. 11. Walker MC, Smith GN, Perkins SL, Keely EJ, Garner PR. Changes in homocysteine levels during normal pregnancy. Am J Obstet Gynecol 1999; 180:660 – 4. 12. den Heyer M, Brouwer IA, Bos GM, Blom HJ, van der Put NMJ, Spaans AP, et al. Vitamin supplementation reduces blood homocysteine levels: a controlled trial in patients with venous thrombosis and healthy volunteers. Arterioscler Thromb Vasc Biol 1998;18:356 – 61.

Effect of Storage on Phenylalanine and Tyrosine Measurements in Whole-Blood Samples, Martin Beck, Arend Bo¨kenkamp,* Nicolas Liappis, and Michael J. Lentze (The Children’s Hospital, Medical Center of Bonn University, D-53113 Bonn, Germany; * address correspondence to this author at: Universita¨tskinderklinik Bonn, Adenauerallee 119, D-53113 Bonn, Germany; fax 49-228-287-3444, e-mail [email protected]) With an incidence of 1 in 6600 newborns, phenylketonuria (PKU) is among the most common inborn errors of metabolism. PKU is caused by a deficiency of hepatic phenylalanine hydroxylase (1 ). The increase in the blood

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Phe concentration leads to permanent structural damage of the central nervous system as a result of disturbed myelination and neurotransmitter deficiency (2 ). If plasma Phe concentrations are normalized by a lowprotein diet with supplementation of essential amino acids before 3 weeks of age, irreversible mental retardation is prevented (2 ). Still, strict metabolic control is mandatory throughout childhood (2 ) and perhaps into adult life (3 ). This is achieved by regular measurement of blood Phe and Tyr concentrations. Tyr is monitored because Phe hydroxylase deficiency renders it an essential amino acid in PKU. Recommendations for the duration and intensity of dietary control are not uniform (2 ); likewise, there is some variation in the local practices of PKU monitoring. The current guidelines of the German “Arbeitsgemeinschaft fu¨r Pa¨diatrische Stoffwechselerkrankungen” (The German Working Group for Metabolic Diseases) set a target range for Phe concentrations of 40 –250 ␮mol/L, at least until age 10 (ⱕ900 ␮mol/L is acceptable during adolescence) (4 ). These recommendations were based on data from samples that were analyzed immediately after sampling (Udo Wendel, University Children’s Hospital, Du¨sseldorf, Germany, personal communication). Pregnant women with even mild hyperphenylalaninemia also require strict dietary control of Phe concentrations to prevent PKU-induced fetopathy (1 ). To ensure optimal metabolic control, patients are monitored in specialized clinics where dietary Phe intake recommendations are adjusted according to weekly to monthly Phe and Tyr measurements. To reduce the inconvenience of regular outpatient visits, most German metabolic centers (like centers in other countries) have established methods that allow patients to have capillary samples collected at home and mailed to the laboratory as whole blood (5 ). Postal transfer of whole-blood samples takes 24 – 48 h. Storage of whole blood for 6 h at 20 °C has been shown not to significantly affect the recovery of Phe and Tyr (6 ). To our knowledge, a delay of 48 h until sample preparation, which is introduced by the mailing process, has not been formally evaluated. We therefore studied the effect of delayed sample preparation on Phe and Tyr serum concentrations from whole blood. Forty-nine blood samples from 35 PKU patients (11.9 ⫾ 10.1 years of age) were obtained by venipuncture during routine monitoring at the metabolic unit of Bonn University Children’s Hospital. Blood was collected into tubes that contained bead-activating coagulation (Serum Monovette; Sarstedt). After informed consent, Phe and Tyr were measured immediately in one aliquot (Pheearly, Tyrearly), whereas the second aliquot was stored as whole blood at room temperature for 48 h until amino acid analysis (Phelate, Tyrlate), thus simulating the mailing process. Serum sample preparation included centrifugation for 10 min at 500g and deproteinization with 50 g/L sulfosalicylic acid (1:1 by volume). Amino acid analysis was performed by ion-exchange chromatography using a

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Technical Briefs

Fig. 1. Bland-Altman analysis of differences between immediate (early) and delayed (late) measurement of Phe (A) and Tyr (B). Horizontal lines at y ⫽ 0 represent a difference of zero between early and late measurements. Slanted lines represent linear regression between residuals and means: for Phe, y ⫽ 30.84 ⫺ 0.018x (r ⫽ 0.327; P ⫽ 0.022); for Tyr, y ⫽ 8.85 ⫹ 0.008x (r ⫽ 0.06; P ⫽ 0.71).

Pharmacia 4151 alpha plus automated analyzer (LKB) as described previously (7 ). Chromatograms were read using a Shimadzu C-R6A Chromatopac Integrator (Shimadzu). Day-to-day CVs were 6.0% and 2.1% for Phe concentrations of 50 and 330 ␮mol/L, respectively. For Tyr, the CV was 2.7% at a mean concentration of 31 ␮mol/L. Data are presented as means ⫾ SE, and statistical analysis was performed using the Wilcoxon test. To check for a potential concentration effect on deviations between immediate (early) and delayed (late) measurements, Bland-Altman analysis was performed (8 ). Pheearly (350 ⫾ 40 ␮mol/L) and Tyrearly (66 ⫾ 6.5 ␮mol/L) were significantly lower than Phelate (372 ⫾ 40 ␮mol/L) and Tyrlate (75.6 ⫾ 6.5 ␮mol/L), respectively (P ⬍0.0001). Bland-Altman analysis (Fig. 1) showed that the difference between Pheearly and Phelate was inversely related to Phe serum concentrations (r ⫽ 0.327; P ⫽ 0.022). Residuals for Tyr did not correlate with Tyr serum concentrations. In the present study, storage of whole-blood samples at room temperature for 48 h substantially increased the serum concentrations of Phe and Tyr. For Phe, this effect was most prominent at low concentrations. Similar changes have been observed for Phe and Tyr in EDTA plasma (Sabine Scholl, Hannover Medical School Children’s Hospital, Hannover, Germany, personal communication). The increase in amino acid concentration probably reflects proteolysis of proteins and peptides in whole blood and a transfer from blood cells into serum. Studying amino acid transfer between erythrocytes and plasma in vitro, Schaefer et al. (9 ) showed a transfer of Phe and

Tyr along the concentration gradient. Under physiological conditions, Phe and Tyr concentrations were 30 – 40% higher in red blood cells than in plasma. The authors noted that there was an inverse correlation between the changes and the original intracellular amino acid concentrations after incubation of erythrocytes with a solution that contained amino acids at concentrations fourfold higher than physiological concentrations. They postulated that there are intracellular saturation concentrations that cannot be exceeded. This might explain why the increase in Phelate diminished with increasing Phe concentrations in our experiment. This effect was negligible for Tyr because Tyr concentrations are not increased in PKU and therefore the concentration range was much narrower. Do the observed increases in Phe and Tyr concentrations induced by delayed sample preparation challenge the current practice in PKU monitoring? The quality of metabolic control during childhood has been shown to correlate directly with intellectual outcome in PKU patients (10, 11 ). With age, higher Phe concentrations are tolerated without adverse effects on intelligence; however, sustained attention, as well as novel problem-solving capabilities, are inversely correlated with high concurrent Phe concentrations (12, 13 ). In the absence of reliable home monitoring of Phe concentrations, dietary management and patient dietary education has to rely on frequent Phe and Tyr measurements in specialized laboratories (5 ). In this setting, the advantage of frequent short-term measurements in mailed whole-blood samples has to be weighed against the inaccuracy introduced by delayed sample preparation. The mailing process leads to an

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overestimation of Phe and Tyr concentrations, which may cause mild overtreatment, i.e., a reduction in dietary Phe allowances in excess of metabolic tolerance. This is not critical with Phe concentrations at or above the upper limit of normal. Still, neurological outcome has also been demonstrated to be adversely affected by Phe deficiency (14 ). In view of an overestimation by up to 60 ␮mol/L in the low concentration range and a target range of 40 –250 ␮mol/L in fresh samples, it would appear prudent to aim at Phe concentrations ⬎100 ␮mol/L for samples measured after delayed preparation from whole blood. In conclusion, the clinical practice of delayed measurement of Phe and Tyr in whole-blood samples mailed to the specialized laboratory leads to slightly increased readings. This is clinically safe, as long as Phe concentrations are kept above 100 ␮mol/L to prevent Phe deficiency.

We thank Ulla Aberfeld and Elisabeth Salvay for logistic help and Claudia Ullmann and Sonja Gross for performing the amino acid measurements.

References 1. Scriver CR, Kaufman S, Eisensmith RC, Woo SLC. The hyperphenylalaninemias. In: Scriver CR, Beaudet AL, Soy MS, Valle D, eds. The metabolic basis of inherited disease, 7th ed. New York: McGraw-Hill, 1993:1015–75. 2. Burgard P, Rey F, Rupp A, Abadie V, Rey J. Neuropsychologic functions of early treated patients with phenylketonuria, on and off diet: results of a cross-national and cross-sectional study. Pediatr Res 1997;41:368 –74. 3. Cerone R, Schiaffino MC, Di Stefano S, Veneselli E. Phenylketonuria: diet for life or not? Acta Paediatr Scand 1999;88:664 – 6. 4. Burgard P, Bremer HJ, Buhrdel P, Clemens PC, Mo¨nch E, Przyrembel H, et al. Rationale for the German recommendations for phenylalanine level control in phenylketonuria. Eur J Pediatr 1999;158:46 –54. 5. Wendel U. [Phenylketonuria in adolescence: continued low-phenylalanine diet with self-monitoring]. Monatsschr Kinderheilkd 1994;142:122–5. 6. Schaefer A, Piquard F, Haberey P. Plasma amino acid analysis: effects of delayed samples preparation and of storage. Clin Chim Acta 1987;164: 163–9. 7. Liappis N, Gobien N, Schlebusch H. [Reference values for the concentration of free amino acids in fasting blood serum of children]. Klin Padiatr 1990;202:161–7. 8. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;1:307–10. 9. Schaefer A, Piquard F, Haberey P. The effects of changes in plasma amino acid concentrations on erythrocyte amino acid content. Clin Biochem 1990;23:237– 40. 10. Smith I, Beasley MG, Ades AE. Effect on intelligence of relaxing the low phenylalanine diet in phenylketonuria. Arch Dis Child 1991;66:311– 6. 11. Zeman J, Pijackova A, Behulova J, Urge O, Saligova, Hyanek J. Intellectual and school performance in adolescents with phenylketonuria according to their dietary compliance. The Czech-Slovak collaborative Study. Eur J Pediatr 1996;155(Suppl 1):S56 – 8. 12. Schmidt E, Rupp A, Burgard P, Pietz J, Weglage J, de Sonneville L. Sustained attention in adult phenylketonuria: the influence of the concurrent phenylalanine-blood-level. J Clin Exp Neuropsychol 1994;16:681– 8. 13. Ris MD, Williams SE, Hunt MM, Berry HK, Leslie N. Early-treated phenylketonuria: adult neuropsychologic outcome. J Pediatr 1994;124:388 –92. 14. Smith I, Beasley MG, Ades AE. Intelligence and quality of dietary treatment in phenylketonuria. Arch Dis Child 1990;65:472– 8.

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Mean Serum Concentration of Vitamin D-binding Protein (Gc Globulin) Is Related to the Gc Phenotype in Women, Anna Lis Lauridsen,1,2* Peter Vestergaard,3 and Ebba Nexo1 (1 Department of Clinical Biochemistry, AKH, Aarhus University Hospital, DK-8000 Aarhus C, Denmark; 2 Department of Clinical Biochemistry, Randers Centralsygehus, DK-8700 Randers, Denmark; 3 Department of Endocrinology and Metabolism, AAS, Aarhus University Hospital, DK-8000 Aarhus C, Denmark; * author for correspondence: fax 45-89-49-30-60, e-mail [email protected]) Immunonephelometry has been reported (1 ) to be a suitable method for quantification of vitamin D-binding protein (also known as Gc globulin or Gc). We wished to develop such a method and examine the association between the mean serum concentration of Gc in women of known Gc phenotype and the phenotype of Gc. Gc is a 52- to 58-kDa multifunctional plasma protein, synthesized mainly by hepatocytes. Polymorphisms in the Gc gene (codominant alleles) give rise to three major electrophoretic variants of Gc (Gc2, Gc1s, and Gc1f), which differ by amino acid substitutions as well as glycosylation (2, 3 ). The physiological significance related to the various phenotypes is yet to be discovered. Gc is the major carrier protein of vitamin D and its metabolites in the circulation and is important for preservation of the vitamin (4, 5 ). Gc also transports components such as fatty acids and endotoxin (6, 7 ), and it is an important player in the actin scavenging system (8, 9 ). Gc binds actin released from cells upon injury, and the Gc-actin complexes are rapidly cleared from the circulation, thereby preventing the harmful effects of actin filaments in blood vessels. The resulting decrease in Gc concentration makes Gc usable as a prognostic indicator of survival of patients with significant tissue injury after trauma (10 ) and among patients with hepatic failure (11 ). In addition to being a transporter and an actin scavenger protein, Gc may be of importance for bone formation and in the immune system. After in vitro removal of its galactose and sialic acid residues, Gc is converted to a very potent macrophage-activating factor, Gc-MAF (12 ). Administration of Gc-MAF to osteopetrotic rodents reversed their bone and immunological defects, probably by activating osteoclasts as well as macrophages (13 ). Finally, together with complement factors C5a and C5a des Arg, Gc may act as a co-chemotactic factor in facilitating chemotaxis of neutrophils and monocytes in inflammation (14, 15 ). Despite the diverse and important roles of Gc, only a few methods for measurement of the protein in serum have been evaluated, and little attention has been paid to possible variation in the serum concentration of Gc as a function of phenotype. We established an immunonephelometric method for quantification of serum Gc, essentially as described previously by Haughton and Mason (1 ), using a Behring Nephelometer II (Dade Behring) and reagents supplied by Behring (N Reaction Buffer, N Diluent, and N Supplement Reagent Precipitation). Rabbit Anti-Human Gc-Globulin