In vivo effects of high phenylalanine blood levels on Na
+
,K
+
-ATPase,
Mg
2+
-ATPase activities and biogenic amine concentrations in
phenylketonuria
Kleopatra H. Schulpis
a
, Joanna Tjamouranis
a
, George A. Karikas
b
, Helen Michelakakis
a
,
Stylianos Tsakiris
c,
*
a
Institute of Child Health, “Aghia Sophia” Children’s Hospital, GR-11527 Athens, Greece
b
Pharmacokinetics and Parenteral Nutrition Unit, “Aghia Sophia” Children’s Hospital, GR-11527 Athens, Greece
c
Department of Experimental Physiology, Medical School, University of Athens, Athens, Greece
Received 13 March 2002; received in revised form 26 April 2002; accepted 26 April 2002
Abstract
Objective: To evaluate the activities of Na
+
,K
+
-ATPase and Mg
2+
-ATPase in erythrocyte membranes from phenylketonuric (PKU)
patients and to correlate the enzyme activities with their blood phenylalanine (Phe) levels, biogenic amines as well as with their precursors
tyrosine (Tyr) and tryptophan (Try).
Design and methods: Twenty three PKU patients were divided into group A (n = 12) on a stricted diet (Phe 1.57 0.52 mg/dL or 0.10
0.03 mM) and group B (n = 11) on a “loose” diet (Phe 24.45 1.50 mg/dL or 1.72 0.09 mM). The enzyme activities were measured
spectrophotometrically, the amino acids with an automatic amino analyser and the biogenic amines with HPLC methods.
Results: In group B, plasma amino acids (Tyr, Try), their biogenic amines [adrenaline (A), noradrenaline (NA), dopamine (DA) and
serotonin (5HT)], (Na
+
,K
+
)-ATPase and Mg
2+
-ATPase activities were found remarkably decreased (p 0.001).
Conclusions: High Phe and/or low NA, DA, 5HT plasma levels may indirectly inhibit the erythrocyte membrane Na
+
,K
+
-ATPase and
Mg
2+
-ATPase in PKU patients. The observed enzyme inhibitions could be a very informative peripheral marker as regards the neurotoxic
Phe brain effects. © 2002 The Canadian Society of Clinical Chemists. All rights reserved.
Keywords: Na
+
,K
+
-ATPase; Mg
2+
-ATPase; Biogenic amines; Phenylalanine blood levels; Phenylketonuria; Erythrocyte membranes
1. Introduction
Phenylketonuria represents a spectrum of recessively in-
herited metabolic disorders where the conversion of the
aromatic amino acid phenylalanine (Phe) to tyrosine (Tyr),
which is the precursor of catecholamines, is impaired. Clas-
sical phenylketonuria (PKU) is an inborn error of metabo-
lism treated by a low Phe diet started, in the very first few
days of life. This measure prevents the mental retardation,
seizures and eczema, which are the main clinical manifes-
tations in untreated patients [1–3].
Unfortunately, many PKU patients do not adhere strictly
to this diet (on a “loose” diet), resulting in uncontrolled high
Phe blood levels [1,3]. As a consequence, high Phe levels
interfere with the production of the neurotransmitters dopa-
mine (DA) and noradrenaline (NA) [4]. Moreover, Krause
et al. [5] observed an inverse relationship between DA
excresion and plasma Phe levels. Blau [6] also reported an
impairment in the neurotransmitter amine synthesis due to
high Phe levels. Phe decreases the availability of tryptophan
(Try) and Tyr [7] and causes serotonin (5HT) and catechol-
amine depletion in PKU [8] thus influencing the brain func-
tions. On the other side, (Na
+
,K
+
)-ATPase (E.C. 3.6.1.3) a
membrane bound enzyme responsible for its active Na
+
and
K
+
transportation across the cell membrane [9], is essential
for synaptic and cellular functions and responsible for brain
development [10]. In our previous in vitro studies [11,12],
we found a concentration dependent decrease of the enzyme
activity on rat diaphragm and some rat brain areas, when
their homogenates were incubated for 1 h with high con-
centrations of Phe. On the contrary, Mg
2+
- ATPase was not
influenced by Phe in vitro. Additionally, Rajanna et al. [13]
* Corresponding author. Fax: +003-010-7775295.
E-mail address: stsakir@cc.uoa.gr (S. Tsakiris).
Clinical Biochemistry 35 (2002) 281–285
0009-9120/02/$ – see front matter © 2002 The Canadian Society of Clinical Chemists. All rights reserved.
PII: S0009-9120(02)00311-9