Degradation of rat liver cytochromes P450 3A after their inactivation by 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine: characterization of the proteolytic system.

The suicide substrate 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4- dihydropyridine (DDEP) inactivates rat liver cytochrome P450 (P450) 3A isozymes through prosthetic heme alkylation of the apoprotein in a mechanism-based fashion, which marks them for rapid proteolysis. In this article, through the use of epitope-specific monoclonal antibodies, we show that both 3A1 and 3A2 isozymes are targeted for proteolysis. Furthermore, using intact rats, isolated rat hepatocytes, and rat liver subcellular fractions supplemented with ATP and MgCl2, as well as various proteolytic inhibitors as probes, we now report that the hepatic cytosolic ubiquitin-dependent proteolytic system rather than hepatic lysosomes is involved in the rapid degradation of DDEP-induced heme alkylated P450s 3A.     

Models of membrane resonance in pigeon semicircular canal type II hair cells

Pigeon vestibular semicircular canal type II hair cells often exhibit voltage oscillations following current steps that depolarize the cell membrane from its resting potential. Currents active around the resting membrane potential and most likely responsible for the observed resonant behavior are the Ca⁺⁺-insensitive, inactivating potassium conductance I _A (A-current) and delayed rectifier potassium conductance I _K. Several equivalent circuits are considered as representative of the hair cell membrane behavior, sufficient to explain and quantitatively fit the observed voltage oscillations. In addition to the membrane capacitance and frequency-independent parallel conductance, a third parallel element whose admittance function is of second order is necessary to describe and accurately predict all of the experimentally obtained current and voltage responses. Even though most voltage oscillations could be fitted by an equivalent circuit in which the second order admittance term is overdamped (i.e., represents a type of current with two time constants, one of activation and the other of inactivation), the sharpest quality resonance obtained with small current steps (around 20 pA) from the resting potential could be satisfactorily fit only by an underdamped term.