Human carnitine deficiency occurs as (a) a primary genetic disease (systemic carnitine deficiency, myopathic carnitine deficiency), (b) in infants due to inadequate carnitine synthesis and/or renal reabsorption in the absence of dietary carnitine, or (c) as a secondary consequence of a variety of genetic or acquired diseases. To date, factors which regulate carnitine biosynthesis, absorption and metabolism in humans are not well understood. In the proposed studies, precursor-loading will be used to determine the rate-limiting factor(s) for carnitine biosynthesis in adult humans. Using radiotracer and kinetic techniques, rates and/or extent of carnitine absorption and intestinal degradation will be measured, and the metabolic fate of absorbed carnitine will be explored in human subjects. These parameters will be examined for three dietary conditions: high-carnitine diet, low-carnitine diet, and therapeutic carnitine loading. These studies will demonstrate the normal mechanisms by which dietary carnitine is utilized by the body, and the efficiency of such utilization under different dietary conditions. The etiology of hepatic gamma-butyrobetaine hydroxylase deficiency in three fetal cases of systemic carnitine deficiency will be studied by (a) purification and characterization of hepatic and renal gamma-butyrobetaine hydroxylase enzymes from normal tissues, and (b) qualitative and quantitative immunochemistry of patient liver with antibodies to the hepatic enzyme. Systemic carnitine deficiency is associated with fluctuating plasma carnitine levels, increased excretion of medium-chain dicarboxylic acids (in some individuals) and increased ratio of esterified to free carnitine in urine. Using high-performance liquid chromatography, radioenzymatic assays and mass spectrometry, we will describe the temporal relationships between these parameters and identify specific acylcarnitine esters in urine. Results may suggest a cause-and-effect relationship between rises in plasma carnitine (implicating loss from tissues) and increases in medium chain dicarboxylic acid and acylcarnitine ester production and excretion. These correlations may provide a means to differentiate primary systemic carnitine deficiency from genetic defects in fatty acyl-CoA dehydrogenation by non-invasive means. These studies will include analyses of normal urine and urine from four systemic carnitine deficiency patients, collected under conditions of controlled caloric and carnitine intake.
