AJP - Gastrointestinal and Liver Physiology, Vol 263, Issue 1 81-G86, Copyright © 1992 by American Physiological Society
Uptake of biotin by isolated rat liver mitochondria
H. M. Said, L. McAlister-Henn, R. Mohammadkhani and D. W. Horne
Medical Research Service, Veterans Affairs Medical Center, Long Beach 90822.
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 approximately 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 microM) and high (8 and 30 microM)
concentrations of biotin. At incubation buffer pH 6.1 and 7.4, uptake of
biotin as a function of concentration (0.012-30 microM) 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.(ABSTRACT TRUNCATED AT 250 WORDS)
