HTML is then cleaved by HTML aldolase (a pyridoxal phosphate requiring enzyme), yielding 4-trimethylaminobutyraldehyde (TMABA) and glycine. TMABA is then dehydrogenated into gamma-butyrobetaine in an NAD+-dependent reaction, catalyzed by TMABA dehydrogenase. Gamma-butyrobetaine is then hydroxylated by gamma butyrobetaine hydroxylase (a zinc binding enzyme) into l-carnitine, requiring iron in the form of Fe2+.Carnitine is involved in transporting fatty acids across the mitochondrial membrane, by forming a long chain acetylcarnitine ester and being transported by carnitine palmitoyltransferase I and carnitine palmitoyltransferase II. Carnitine also plays a role in stabilizing Acetyl-CoA and coenzyme A levels through the ability to receive or give an acetyl group. Tissue distribution of carnitine-biosynthetic enzymes
The tissue distribution of carnitine-biosynthetic enzymes in humans indicates TMLD to be active in the liver, heart, muscle, brain and highest in the kidney. HTMLA activity is found primarily in the liver. The rate of TMABA oxidation is greatest in the liver, with considerable activity also in the kidney. Carnitine shuttle system
The free-floating fatty acids, released from adipose tissues to the blood, bind to carrier protein molecule known as serum albumin that carry the fatty acids to the cytoplasm of target cells such as the heart, skeletal muscle, and other tissue cells, where they are used for fuel.
Metabolic defects of fatty acid oxidation
More than 20 human genetic defects in fatty acid transport or oxidation have been identified. In case of fatty acid oxidation defects, acyl-carnitines accumulate in mitochondria and are transferred into the cytosol, and then into the blood. Plasma levels of acylcarnitine in newborn infants can be detected in a small blood sample by tandem mass spectrometry.When β oxidation is defective because of either mutation or deficiency in carnitine, the ω (omega) oxidation of fatty acids becomes more important in mammals. Actually, the ω Oxidation of Fatty Acids is another pathway for F-A degradation in some species of vertebrates and mammals that occurs in the endoplasmic reticulum of the liver and kidney, it is the oxidation of the ω carbon—the carbon most far from the carboxyl group (in contrast to 
  
    
      
        β
      
    
    {\displaystyle \beta }
   oxidation which occurs at the carboxyl end of fatty acid, in the mitochondria). Physiological effects
As an example of normal synthesis, a 70 kilograms (150 lb) person would produce 11–34 mg of carnitine per day. Adults eating mixed diets of red meat and other animal products ingest some 60–180 mg of carnitine per day, while vegans consume about 10–12 mg per day. Most (54–86%) carnitine obtained from the diet is absorbed in the small intestine before entering the blood.