Uptake of biotin by isolated rat liver mitochondria

H. M. Said, L. McAlister-Henn, R. Mohammadkhani, D. W. Horne

Research output: Contribution to journalArticlepeer-review

11 Scopus citations

Abstract

This study examined the mechanism of biotin uptake by liver mitochondria. Mitochondria were isolated from rat liver by an established procedure and demonstrated normal respiratory control ratios. Uptake of biotin with time at incubation buffer pH 6.1 was rapid and linear and occurred with a distinct 'overshoot' phenomenon that peaked at ~ 1 min of incubation. At incubation buffer pH 7.4, however, uptake of biotin with time was significantly slower and no overshoot was observed. Gradual lowering of incubation buffer pH from 7.9 to 6.1 caused a similar pattern of increase in uptake of low (0.024 μM) and high (8 and 30 μM) concentrations of biotin. At incubation buffer pH 6.1 and 7.4, uptake of biotin as a function of concentration (0.012-30 μM) was linear and occurred at rates of 3.62 and 1.90 pmol · mg protein-1 · 5 s- 1, respectively. Addition to the incubation medium of high concentrations of unlabeled biotin, its related compounds (biocytin, desthiobiotin, biotin methyl ester, thioctic acid, and thioctic amide), or substrates of known mitochondrial transporters (succinate, pyruvate, glutamate, malate, and phosphate) failed to cause any significant inhibition in uptake of [3H]biotin by mitochondria incubated in buffer pH 6.1 and 7.4. Furthermore, preloading mitochondria with unlabeled biotin, biocytin, malate, or aspartate failed to cause any significant stimulation in biotin uptake. At incubation buffer pH 6.1, treatment of mitochondria with the protonophore FCCP caused significant inhibition in pH-dependent overshoot of biotin uptake. However, treatment of mitochondria with the potassium ionophore valinomycin caused significant stimulation in the pH-dependent overshoot of biotin uptake. The uptake process of biotin was also found to be temperature dependent. Results indicate that biotin uptake by mitochondria is via a pH-dependent non- mediated simple diffusion process. It is postulated that biotin enters the intramitochondrial space in the neutral protonated form and dissociates into the anionic form at alkaline mitochondrial pH, becoming entrapped within.

Original languageEnglish (US)
Pages (from-to)G81-G86
JournalAmerican Journal of Physiology - Gastrointestinal and Liver Physiology
Volume263
Issue number1 26-1
DOIs
StatePublished - 1992

Keywords

  • 'overshoot' phenomenon
  • pH-dependent diffusion

ASJC Scopus subject areas

  • Physiology
  • Hepatology
  • Gastroenterology
  • Physiology (medical)

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