Effect of detergents on in vitro 7α-hydroxycholesterol formation by rat liver microsomes

Formation of 7α-hydroxycholesterol by rat liver microsomes was quantitated using a gas chromatograph-mass spectrometer (GC/MS) operated in selected ion monitoring (SIM) mode. Microsomes from normal rat livers incubated for different periods were found to yield increased 7α-hydroxycholesterol with time. This was also true when incubations contained Tween-80, but in this instance, the rate of 7α-hydroxycholesterol production was lower and dependent on the concentration of Tween used. Similarly, Triton X-100, Renex-30, Kyro EOB, Cutscum, and Emulgen 911 all lowered the formation of 7α-hydroxycholesterol by rat liver microsomes, whereas Triton WR-1339 stimulated its production. Analysis of data obtained from following the enzyme reaction over an extended period using an integrated Michaelis-Menten equation indicated the enzyme possesses a very significant affinity for the product (K_s>K_p). Similar analysis shows that Tween-80 is a noncompetitive inhibitor of the enzyme.     

Fluorescent probing of membrane potential in walled cells: diS-C3(3) assay in Saccharomyces cerevisiae

Membrane-potential-dependent accumulation of diS-C3(3) in intact yeast cells in suspension is accompanied by a red shift of the maximum of its fluorescence emission spectrum, lambda max, caused by a readily reversible probe binding to cell constituents. Membrane depolarization by external KCl (with or without valinomycin) or by ionophores causes a fast and reproducible blue shift. As the potential-reporting parameter, the lambda max shift is less affected by probe binding to cuvette walls and possible photobleaching than, for example, fluorescence intensity. The magnitude of the potential-dependent red lambda max shift depends on relative cell-to-probe concentration ratio, a maximum shift (572-->582 nm) being found in very thick suspensions and in cell lysates. The potential therefore has to be assessed at reasonably low cell (< or = 5 x 10(6) cells/ml) and probe (10(-7)M) concentrations at which a clearly defined relationship exists between the lambda max shift and the potential-dependent accumulation of the dye in the cells. The redistribution of the probe between the medium and yeast protoplasts takes about 5 min, but in intact cells it takes 10-30 min because the cell wall acts as a barrier, hampering probe penetration into the cells. The barrier properties of the cell wall correlate with its thickness: cells grown in 0.2% glucose (cell wall thickness 0.175 +/- 0.015 micron, n = 30) are stained much faster and the lambda max is more red-shifted than in cells grown in 2% glucose (cell wall thickness 0.260 +/- 0.043 micron, n = 44). At a suitable cell and probe concentration and under standard conditions, the lambda max shift of diS-C3(3) fluorescence provides reliable information on even fast changes in membrane potential in Saccharomyces cerevisiae.     

A strategy for the numerical evaluation of Fourier sine and cosine transforms to controlled accuracy

A strategy for the numerical evaluation of Fourier sine and cosine transforms is presented. The method is capable of high accuracy and incorporates error estimates which enable one to control the accuracy to which a computation is to be performed. Illustrative applications to a simple test case and to a non-trivial problem taken from the recent physics literature are given.     

Development of a multitechnology field-programmable gate array suitable for photonic information processing

The architecture of a novel, multitechnology field-programmable gate array (FPGA) is introduced. Based on conventional complementary metal-oxide semiconductor VLSI technology this architecture has demonstrated the feasibility of reconfigurable and programmable hardware for prototyping photonic information processing systems. We report that this new FPGA architecture will enable the design of reconfigurable systems that incorporated technologies outside the traditional electronic domain. The smart photoreceivers monolithically integrated in the new FPGA architecture can receive optically encoded signals in parallel and process them with user programmable logic hardware.     

Analysis of multiple steady states of complex biochemical reactions

A general theory for the identification of criteria within which an enzymic reaction can exhibit multiplicity (i.e. multiple steady states) is reviewed. Application of the theory to four types of kinetic models encountered in biochemical reactions is illustrated and figures are presented to delineate the region of multiplicity. The stability of the steady states is analysed for small perturbation about the steady state.     

An Adjustable-Speed Induction Motor Drive with a Cycloconverter-Type Thyristor-Commutator in the Rotor

An adjustable-speed drive comprising a slip-ring induction motor and a cycloconverter-type thyristor-commutator in its rotor circuit has been developed. The thyristor-commutator acts as a frequency-changer like a conventional commutator in a stator-fed ac commutator motor, converting line-frequency injected voltages directly to slip-frequency at the rotor terminals. The circuit permits a reversible power flow naturally, and speed-control is possible for subsynchronous as well as supersynchronous operation by controlling the injected secondary voltage. A rotor position-detector is used to switch the thyristor configuration in a sequential manner to generate an output voltage having a predominant slip-frequency component. The control logic and triggering circuit developed for the experimental drive are described in some detail. Experimental results as obtained with a laboratory model are presented together with some results of computer simulation of the drive system. Major limitations of the scheme and the scope for improvement are discussed.