Comparison of α-methylphenylalanine and p-chlorophenylalanine as inducers of chronic hyperphenylalaninaemia in developing rats

Abstract

alpha-Methylphenylalanine is a very weak competitive inhibitor of rat liver phenylalanine hydroxylase in vitro but a potent suppressor in vivo. The loss of the hepatic activity (the renal one is unaffected) becomes maximal (70-75% decrease; cf. control) 18h after the administration (per 10g body wt.) of 24 mumol of alpha-methylphenylalanine with or without 52 mumol of phenylalanine. Chronic suppression of hepatic phenylalanine hydroxylase was obtained by injections of alpha-methylphenylalanine plus phenylalanine to suckling rats, and by their addition to the diet after weaning. A series of comparisons of the effects of this treatment, and one with p-chlorophenylalanine, was then carried out. In both cases there was a rise (1.3-2-fold) in phenylalanine-pyruvate amino-transferase activity (but no change in four other enzyme activities) in the liver; in brain there was a rise in phosphoserine phosphatase activity, but the total activity and subcellular distribution of nine enzymes revealed no other abnormalities in cerebral development. Striking increases in the concentration of plasma phenylalanine during 26 of the 31 experimental days (with a transient fall at 18-22 days) were maintained by treatment with both analogues plus phenylalanine. However, p-chlorophenylalanine-treated animals had a 30-60% mortality rate and 27-52% decrease in body weight. Developing rats treated with alpha-methylphenylalanine, showing no growth deficit or signs of toxicity (e.g. cataracts), appear to be a more suitable model for the human disease of phenylketonuria. Their phenylalanine concentrations exhibited at least 20-40-fold increase during 50% of each of the first 18 days of life, and 30-fold after weaning.

Biochemical, Metabolic, and Behavioral Characteristics of Immature Chronic Hyperphenylalanemic Rats

“…Procedures were similar to those previously described [12, 18, 19] and used either αMePhe or pCPhe to inhibit liver phenylalanine hydroxylase along with large doses of Phe in the presence or absence of enzyme inhibition. Stock solutions (all were adjusted to pH 7.2) for injection of αMePhe (70 μmol/mL), αMePhe (70 μmol/mL) plus Phe (152 μmol/mL), or Phe (152 μmol/mL) were prepared in 0.9% NaCl (w/v) with warming.…”

“…To better understand the biochemical, developmental, physiological, and behavioral changes associated with hyperphenylalanemia, a number of animal models have been developed by treatments with their onset at various postnatal ages for different durations using specific metabolites of Phe given alone (e.g., phenylacetate [9, 10]), or different doses of Phe either alone [11] or in combination with an inhibitor of Phe hydroxylase (p-chlorophenylalanine (pCPhe) or α-methylphenylalanine (αMePhe) [12]). A PKU mouse model (BTBR background-Pah enu2 ) was subsequently developed by producing germline mutagenesis with ethylnitrosourea (enu) and identification of Phe hydroxylase deficiency by Phe clearance screening [13].…”

“…A PKU mouse model (BTBR background-Pah enu2 ) was subsequently developed by producing germline mutagenesis with ethylnitrosourea (enu) and identification of Phe hydroxylase deficiency by Phe clearance screening [13]. These models reproduce many of the biochemical, developmental, and behavioral characteristics of PKU patients, but the use of enzyme inhibitors involves side effects of the drugs and differential toxicity, with pCPhe having greater deleterious effects than αMePhe or high doses of Phe alone, and the residual Phe hydroxylase activity in the rat models causes high levels of tyrosine (e.g., [12, 14–16]). The Phe hydroxylase-deficient mouse model is generally considered advantageous due to lack of inhibitor side effects, but lack of sufficient demand led Jackson Laboratory to maintain the mutant lines by cryopreservation, increasing the cost and time to obtain the animals unless they are available from active PKU researchers [17].…”

Hyperphenylalaninemia in the hph-1 Mouse Mutant

“…Although rhemically induced animal modeling systems for HPA have been developed (5,6,17), there is always a degree of uncertainty inherent in their use due to the potential presence of unknown secondary effects induced by the chemical agents used (7).…”

Phenylalanine Uptake in Neonatal and Infant Rat Brain

“…In the experiments of figure 1, this treatment was given to every rat once (on the indicated day of age) and most of them received no other treatment. A subgroup was also given a daily injection of a-Mcphe plus Phe (replaced by dietary sup plementation at weaning) so as to impose chronic hyperphenylalaninemia from the 3rd day on [ 16], However, this condition did not alter the developmental change in the extent of rise of cerebral Phe content at blood levels elevated to 4.000 nmol/ml.…”