Aminotransferases AMINOTRANSFERASES Definition Interconversion of amino acids to

Aminotransferases

AMINOTRANSFERASES • Definition – Interconversion of amino acids to 2 -oxo-acids – Amino group donor – Amino group acceptor • Aminotransferases of clinical interest • Aspartate aminotransferase (AST) • Alanine aminotransferase (ALT)

• Distribution – In serum • Apoenzymes (Coenzyme-deficient) • Holoenzymes – In tissues • Throughout the body • AST – Primarily » Heart, liver, skeletal muscle, and kidney – Forms » Mitochondrial » Cytoplasmic • ALT – Primarily » Liver and kidney – Exclusively cytoplasmic

Tissue distribution

• ALT is the more liver-specific enzyme • Elevations of ALT activity persist longer than do those of AST activity

Clinical Significance • Liver disease – The most important cause – Tenfold to fortyfold elevations of AST and ALT • Viral hepatitis • Acute hepatic necrosis – To diagnose chronic hepatitis • Persistence of increased ALT for more than 6 months • Most have maximum ALT less than seven times the upper reference

Clinical Significance • Toxic hepatitis • Acetaminophen-induced hepatic injury • Transaminase 85 times the upper reference limit in 90% of case – AST and ALT » Peak early and fall rapidly

• Carcinoma of the liver – Twofold to fivefold elevations – AST usually being higher than ALT • Various medications – Slight or moderate elevations of both AST and ALT • Less common causes – – – Hemochromatosis Wilson's disease Autoimmune hepatitis Primary biliary cirrhosis α 1 -antitrypsin deficiency

Clinical Significance • Increased AST activity – AMI – Progressive muscular dystrophy • Up to eight times normal • With serum CK • Mitochondrial AST (m-AST) – Extensive liver cell degeneration and necrosis – Ratio between m-AST and total AST • Typical of alcoholic hepatitis • Macro-AST – No known clinical relevance

Methods for the Measurement of Transaminase Activity • The assay system – Two amino acids and two oxo-acids • Formation or consumption of the oxo-acids is measured • Various photometric substrates and dyes coupled to the transaminase reactions – Continuous-monitoring methods • • • Transaminase reactions Oxo-acids formed Dehydrogenase reactions Reduction to hydroxy acids Change in NADH

• Supplementation with P-5' –p • NADH, MD or LD, must be present in sufficient quantity • A preliminary incubation – Endogenous oxo-acids • AST activity in serum – Up to 48 hours at 4°C – Frozen if they are to be kept longer • ALT activity should be assayed on the day of sample collection

• ALT stability – at -70°C. • Hemolyzed specimens • Reference Intervals – AST (upper reference limits for adults) • 31 U/L for women • 35 U/L for men – ALT • 34 U/L • 45 U/L

• m-AST – 5% to I 0% of the activity of total AST in serum – Reference limit • 3. 0 U/L