We present absolute measurements of the rate constant for the reactions of OH with cyclohexane: k₁=(8.6±0.8) × 10⁻¹² cm³ molecule⁻¹s⁻¹ and with ethane: k₃= (2.74±0.3) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, both measured at room temperature by discharge flow resonance fluorescence. Our result for k₁ is above the average of two previously published measurements, but is in agreement with the preferred values of two recent reviews, as deduced from either relative measurements or theoretical correlations.

Transport of biotin was examined in rat intestine using the everted sac technique. Transport of 0.1 microM biotin was linear with time for at least 30 min of incubation and occurred at a rate of 3.7 pmol X g initial tissue wet wt-1 X min-1. Transport of biotin was higher in the jejunum than the ileum and was minimum in the colon (85 +/- 6, 36 +/- 6, and 2.8 +/- 0.6 pmol X g initial tissue wet wt-1 X 25 min-1, respectively). In the jejunum, transport of biotin was saturable at low concentrations (Kt = 3.73, microM, Vmax = 3.11 nmol X g initial tissue wet wt-1 X 25 min-1) but linear at higher concentrations (greater than 10 microM). The transport of low concentrations of biotin was inhibited by structural analogues (desthiobiotin, biotin methyl ester, diaminobiotin, and biocytin), Na+ dependent, energy dependent, temperature dependent, and proceeded against a concentration gradient in the serosal compartment. No metabolic alteration occurs to the biotin molecule during transport. This study demonstrates that biotin transport in rat intestine occurs by a carrier-mediated process at low concentrations and by simple diffusion at high concentrations. Furthermore, the carrier-mediated process is Na+, energy, and temperature dependent.