On the uptake of biotin by the rat renal tubule

Imaging Biotin Trafficking In Vivo with Positron Emission Tomography

The water-soluble vitamin biotin is essential for cellular growth, development, and well-being, but its absorption, distribution, metabolism, and excretion are poorly understood. This paper describes the radiolabeling of biotin with the positron emission tomography (PET) radionuclide carbon-11 ([¹¹C]biotin) to enable the quantitative study of biotin trafficking in vivo. We show that intravenously administered [¹¹C]biotin is quickly distributed to the liver, kidneys, retina, heart, and brain in rodents-consistent with the known expression of the biotin transporter-and there is a surprising accumulation in the brown adipose tissue (BAT). Orally administered [¹¹C]biotin was rapidly absorbed in the small intestine and swiftly distributed to the same organs. Preadministration of nonradioactive biotin inhibited organ uptake and increased excretion. [¹¹C]Biotin PET imaging therefore provides a dynamic in vivo map of transporter-mediated biotin trafficking in healthy rodents. This technique will enable the exploration of biotin trafficking in humans and its use as a research tool for diagnostic imaging of obesity/diabetes, bacterial infection, and cancer.

Biotin dependency due to a defect in biotin transport

We describe a 3-year-old boy with biotin dependency not caused by biotinidase, holocarboxylase synthetase, or nutritional biotin deficiency. We sought to define the mechanism of his biotin dependency. The child became acutely encephalopathic at age 18 months. Urinary organic acids indicated deficiency of several biotin-dependent carboxylases. Symptoms improved rapidly following biotin supplementation. Serum biotinidase activity and Biotinidase gene sequence were normal. Activities of biotin-dependent carboxylases in PBMCs and cultured skin fibroblasts were normal, excluding biotin holocarboxylase synthetase deficiency. Despite extracellular biotin sufficiency, biotin withdrawal caused recurrent abnormal organic aciduria, indicating intracellular biotin deficiency. Biotin uptake rates into fresh PBMCs from the child and into his PBMCs transformed with Epstein Barr virus were about 10% of normal fresh and transformed control cells, respectively. For fresh and transformed PBMCs from his parents, biotin uptake rates were consistent with heterozygosity for an autosomal recessive genetic defect. Increased biotin breakdown was ruled out, as were artifacts of biotin supplementation and generalized defects in membrane permeability for biotin. These results provide evidence for a novel genetic defect in biotin transport. This child is the first known with this defect, which should now be included in the identified causes of biotin dependency.

Disturbances in biotin metabolism in children undergoing long-term anticonvulsant therapy

BACKGROUND

In subjects undergoing long-term therapy with carbamazepine and/or phenytoin, reduced plasma concentrations of biotin have been reported. However, the diagnostic value of plasma biotin is unclear, in part because of the presence of significant plasma concentrations of biotin metabolites. Pathologic organic aciduria has also been reported with long-term anticonvulsant therapy, suggesting biotin deficiency, but no mechanism leading to deficiency has yet been determined.

METHODS

In the current study, we sought to determine whether biotin catabolism was accelerated in children receiving long-term treatment with certain anticonvulsants and to assess biotin status as judged by urinary excretion of biotin and 3-hydroxyisovaleric acid, an organic acid that is an indicator of deficiency of a biotin-dependent enzyme. Seven children treated with carbamazepine and/or phenytoin and six treated with phenobarbital provided untimed urine samples. Sixteen healthy children receiving no anticonvulsants served as controls. Biotin and biotin metabolites were determined by high-performance liquid chromatography/avidin-binding assay. Urinary excretion of 3-hydroxyisovaleric acid was determined using gas chromatography/mass spectrometry.

RESULTS

Bisnorbiotin excretion was increased significantly in the carbamazepine/phenytoin group and in the phenobarbital group. Biotin sulfoxide excretion was significantly increased in the carbamazepine/phenytoin group but not in the phenobarbital group. 3-Hydroxyisovaleric acid excretion was increased significantly in the carbamazepine/phenytoin group. However, only one child (carbamazepine/phenytoin group) had a decreased urinary excretion of biotin.

CONCLUSION

These data provide evidence that long-term administration of some anticonvulsants can accelerate biotin catabolism, but the indicators of biotin status conflict.

Biotin transport in primary culture of astrocytes: effect of biotin deficiency

The uptake of radioactive biotin has been studied in glial cell cultures from dissociated cerebral hemispheres of newborn rats. We describe saturable kinetics for the biotin uptake at biotin concentrations of less than 60 nM and linear at greater than 60 nM. The uptake appeared temperature sensitive, Na+ independent, nonsensitive to valinomycin, and not affected by metabolic inhibitors such as sodium fluoride or azide. Lipoic acid and biocytin were effective in inhibiting the biotin uptake. These findings are consistent with biotin uptake by the primary culture of astrocytes as a process of facilitated diffusion. Moreover, biotin uptake in astrocytes grown in biotin-restricted conditions was significantly higher compared with the control. This increase appeared mediated through a pronounced increase (10-fold) in the Vmax of the biotin uptake without any change in the apparent Km.

Biotin uptake in cultured hepatocytes from normal and biotin-deficient rats

Biotin uptake was studied in isolated cultured hepatocytes of normal and biotin-deficient rats. Biotin uptake was temperature-dependent with respect to physical, but not to chemical, processes, proportional to the exogenous biotin concentration in the medium, independent of pH and sodium ion concentrations of the medium, and uneffected by the presence of structural analogues of biotin or metabolic inhibitors in both normal and biotin-deficient hepatocytes. These results suggest that biotin uptake occurs by a passive, nonmediated, non-energy-dependent mechanism in rat hepatocytes.