Disorders of aromatic amino acid metabolism (Phenylalanine, tyrosine) Dr. Ketki K Assistant Professor Department of Biochemistry

Inborn error of Phenylalanine & tyrosine metabolism (IEM)

IEM Cause: Mutant genes results in Abnormal enzyme/ Total loss of enzyme activity/ Partial deficiency of enzyme Further leads to accumulation of substrate & No product formation

IEM of Phe & Tyr Phenylketonuria (PKU) Alkaptonuria Tyrosinemia Albinism

PKU Most common IEM, Autosomal Recessive Incidence- 1: 10,000 Types: Type I- Classical: deficiency of Phenylalanine hydroxylase Type II & III: deficiency of Dihydro biopterin reducase Type IV & V : deficiency of enzyme synthesizing biopterin Elevated blood phenylalanine may not be detectable until 3–4 days postpartum. False-positives in premature infants may reflect delayed maturation of enzymes of phenylalanine catabolismphenylalanine, which becomes a major donor of amino groups in aminotransferase activity and depletes neural tissue of a-ketoglutarate. This absence of a-ketoglutarate in the brain shuts down the TCA cycle and the associated production of aerobic energy, which is essential to normal brain development. phenylpyruvic acid, is reduced to phenylacetate and phenyllactate, and all 3 compounds appear in the urine. The presence of phenylacetate in the urine imparts a "mousy" odor. deficiencies in DHPR can manifest with hyperphenylalaninemia. However, since tetrahydrobiopterin is a cofactor in several other enzyme catalyzed reactions (e.g. see the synthesis of the tyrosine- and tryptophan-derived neurotransmitters as well as nitric oxide in Specialized Products of Amino Acids), the effects of missing or defective DHPR cause even more severe neurological difficulties than those usually associated with PKU caused by deficient phenylalanine hydroxylase activity.

BH4 required for Phenylalanine hydroxylase = synthesis of tyrosine tyrosine hydroxylase = synthesis of catecholamines(NT) tryptophan hydroxylase = synthesis of Serotonin Deficiency of BH4 leads to ↓ tyrosine, dopamine,serotonin in brain

Consequences/biochemical abnormalities

1) Hyperphenylalaninemia (>20 mg/dL) ↑ phenylalnine in tissue, plasma,urine 2)↑phenyllactate,phenylacetate,phenylpyruvate excreted in urine These metabolites give urine : "mousy" odor.

C/F Mental Retardation (low IQ<50),intellectual disability (low IQ),developmental delay, microcephaly, hyperactivity,seizures,agitation Hypopigmentation (elevated phenylalanine competitively inhibits tyrosinase & impairs melanin formation) that causes light skin color,fair hair, blue eyes etc Mousy body odor (due to presence of phenyllactic acid in sweat)

Laboratory diagnosis & screening Blood phenylalanine: Normal level 1 mg/dL In PKU, level > 20 mg/dL diagnosed by chromatography, Tandem mass spectroscopy (TMS) Time of blood collection in new born: 24-48 hrs after protein feeding/breast feeding (to avoid false negative results) Mother clears increased phenylalanine in her affected fetus through placenta

2) Ferric chloride test: blue-green 3) Guthrie’s test: blood/urine: b 2) Ferric chloride test: blue-green 3) Guthrie’s test: blood/urine: b. subtilis 4) DNPH test : Yellow ppt 5) HPLC 6) Prenatal diagnosis by using : cultured amniotic cells

Treatment 1) Diet low in phenylalanine (tapioca/cassava)/ synthetic amino acid preparation free of phenylalanine supplemented with some natural foods (fruits,vegetables, cereals) selected for their low phenylalanine content Treatment must begin during first 7-10 days of life to prevent neurological impairment) Lifelong restriction of phenylalanine is recommended

2) supply tyrosine In diet 3) supplementation with BH4 & L-3,4-dihydroxyphenylalnine(L-DOPA) & 5- hydroxytryptophan Improves clinical outcome in type II,III.IV, v PKU

Maternal PKU: Lady having PKU,but not on low Phe diet becomes pregnant Then the offspring develops “Maternal PKU syndrome”. It causes microcephaly & congenital heart abnormalities in fetus. It can be prevented by dietary restriction of diet containing phenylalanine in mother prior to pregnancy & maintain throughout pregnancy

Alkaptonuria Autosomal recessive, incidence 1 in 2,50,000 births Metabolic defect: Deficiency of Homogentisic acid oxidase Consequences: increased homogentisic acid in urine Arthritis due to binding of HGA to cartilageThe first genetic disease ever recognized, was demonstrated to be the result of a defect in phenylalanine and tyrosine catabolism. Garrod’s tetrad: alkaptonuria,albinism, pentosuria,cystinuria

Biochemical manifestation

C/F: Patient asymptomatic until age 40 years Urine gets darken on exposure to air Dark staining of diapers indicate disease in infants Large joint arthritis, Ochronosis (deposition of black pigment/alkapton bodies on cartilage & collagenous tissue)

Investigations: Urine: black on standing Ferric chloride test: green color Benedict’s test: orange-red color HPLC

Treatment: Not life threatening condition so no specific treatment required Diet low in phenylalanine & tyrosine

Albinism Autosomal recessive, Incidence: 1 in 20,000 Due to deficiency in synthesis of pigment melanin which is due to absence of tyrosinase (copper containing enzyme) in eye & skin [ c/a tyrosinase negative oculocutaneous albinism ] When tyrosinase or melanin forming cells are absent from epidermis leukoderma results

Hypopigmentation of skin, fundus & iris Associated Photophobia,nystagmus,↓ visual acuity Skin sensitivity to sunlight, prone to skin cancer(melanoma)

Tyrosinemia 1)Type I Tyrosinemia: Hepatorenal -Tyrosenemia 2) Type II Tyrosinemia: Oculocutaneous- Tyrosenemia 3) Type III Tyrosinemia : Neonatal Tyrosenemia 4) Hawkinsinuria

1)Type I Tyrosinemia: Hepatorenal -Tyrosinemia Also c/a tyrosinosis AR Incidence: 1.5 per 1,000 births Cause: def of Fumaryl acetoacetate hydrolase

C/F: Cabbage like odor Hypoglycemia Liver failure, renal tubular dysfunction Vitamin D resistant rickets Mild Mental Retardation, Death may occur in first 6 months of life

Investigation: Urine contains tyrosine, pHPPA,hydroxyphenyllactic acid Serum: shows tyrosine Treatment: Tyrosine & phenylalanine restricted diet

Type II Tyrosinemia: Oculocutaneous- Tyrosinemia C/a Richner-Hanhart syndrome Cause: deficiency of tyrosine transaminase Consequences : urinary excretion of tyrosine,tyramine C/F: Mental retardation, dermatitis, palmoplantar keratosis, photophobia Treatment: diet low in phe,tyr

Type III Tyrosinemia : Neonatal Tyrosinemia Due to def of p-hydroxy phenylpyruvate hydroxylase Temporary condition Overcome by treatment with ascorbic acid

Hawkinsinuria Autosomal dominant Due to def of p-HPPA oxidase Excretion of p-HPPA in urine

Case 1 2 weeks infant: convulsions, mousy odor in urine, Ferric chloride test; green

Plasma phenylalanine ↑increased Urine Phenylalanine, Phenylpyruvate, Phenyllactate ↑

What is the importance of early diagnosis? What is the defect? Why convulsions? Why fair skin? What is the importance of early diagnosis? Convulsion; decreased neurotransmitter synthesis (serotonin) by Ph alaniane + direct toxicity + decreased GABA synthesis Diagnosis: can prevent mental retardation Fair skin: phenylalanine blocks tyrosinase

Case 2 3 yrs child is having white hair + fair skin addition of THB had no effect What is the biochemical defect? Will there be any abnormality of catecholamine metabolism? No effect on catecholamine met as they are forme in adrenal medulla from a totally different enzyme. And melanin inside melanocytes

Case 3 Mother of 1.5 year old child came with complaint of progressive appearance of hyperchromic papules on his second fingers of both hands since 1 year. he also had darkening of urine & diaper. What is the diagnosis?

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