Uptake of
biocytin and
biotin was investigated in cultured transformed variants of neuronal (NB2a
neuroblastoma) and glial (C6
astrocytoma) CNS cells. NB2a cells took up both compounds but
biocytin was transported more efficiently than
biotin in the nanomolar concentration range. In NB2a cells a single transport mechanism was found for
biocytin with different kinetic parameters in the presence of high extracellular Na+ (Km 0.4 microM, Vmax 20 pmol/min/mg), K+ (Km 1.7 microM, Vmax 32 pmol/min/mg), or choline+ (Km 0.1 microM, Vmax 5 pmol/min/mg). Two transport systems (Km1 17 microM, Vmax1 53 pmol/min/mg; Km2 314 microM, Vmax2 360 pmol/min/mg) were identified for
biotin with only system 1 being Na+-dependent.
Biocytin uptake was competitively inhibited by excess
biotin but not vice versa. Inhibition studies with structural analogs indicated different specificities for
biotin and
biocytin uptake.
Biocytin uptake into C6 cells was hardly detectable whereas
biotin was taken up by diffusion (kD 0.6 microl/min/mg) and a single saturable mechanism (Km 70 microM, Vmax 119 pmol/min/mg) at high extracellular Na+. High extracellular K+ enhanced
biotin diffusion into C6 cells. Inhibition studies with structural analogs revealed a less specific
biotin uptake mechanism in C6 than in NB2a cells.
Biocytin normalized deficient
biotin-dependent
propionyl-CoA carboxylase activity within 4 h in
biotin-deficient NB2a cells whereas in C6 cells reactivation was <20% thereby confirming that
biocytin is only poorly transported into C6 cells. Specific
biocytin uptake into NB2a cells is to our knowledge the first demonstration of a carrier-mediated transport mechanism for this compound. Neuronal
biocytin uptake might contribute to the pathogenesis of
biotinidase deficiency where
biocytin is present in elevated levels.