The Contractile and Control Sites of Natural Actomyosin

The various contractile and control sites of natural actomyosin gel were studied by comparing the kinetics of ATP hydrolysis with those of gel contraction, measured as an increase in turbidity. Contraction of actomyosin gel seems to require the cooperative reaction of ATP (with Mg) at two different sites. One of these sites catalyzes the hydrolysis of ATP and most probably contributes the driving force for contraction; the binding of ATP to the other site appears to break certain links that retard movement of the gel components. At limiting concentrations of ATP, the rate of contraction seems to depend on the rate of breaking these links as well as on the rate of ATP hydrolysis. But when both sites are saturated, the rate of contraction appears to be limited only by the rate of ATP hydrolysis. In addition to these two contractile sites, there are also two different control sites. At one, the relaxing site, the binding of ATP with Mg inhibits ATP hydrolysis and gel contraction. At the other, the binding of calcium activates contraction by overcoming the inhibitory action of Mg and ATP at the relaxing site. This control system—inhibition by substrate and disinhibition by calcium—can be selectively inactivated by heat and reactivated by dithiothreitol, a disulfide-reducing agent. These observations on the isolated contractile system are discussed in relation to the contraction and relaxation of muscle.     

METHOD FOR COMBINED ULTRASTRUCTURAL AND BIOCHEMICAL ANALYSIS OF NEURAL TISSUE

No significant change was found in the electrolytes and lipids of the brain analyzed after glutaraldehyde fixation by perfusion of laboratory animals; such fixation also satisfactorily preserves neural tissues for electron microscopy. The brains of normal and tumor-bearing C3H mice, Wistar rats, and New Zealand rabbits were studied. Little difference was found in the dry weight and the content of sodium, potassium, total lipid and lipid fractions, and in the sulfate space (S³⁵O₄) between specimens from unperfused and perfused animals, whether normal or tumor-bearing. The results suggest the possibility of using selected regions of the nervous system, dissected after fixation, for chemical study and at the same time characterizing similar regions morphologically with the electron microscope.     

Use of a triaxial magnetometer for respiratory measurements

We describe a triaxial magnetometer (Tri-mag) system, which consists of a transmitter, four sensors, a processing unit, and a personal computer (PC). The Tri-mag processing unit outputs the position of each sensor relative to the transmitter in three orthogonal coordinates, and this information is communicated to the PC. First, we demonstrated that within a defined octant of a sphere in which the center is the transmitter, we can measure radial distances with an accuracy of +/- 1 mm over a range extending from 10 to 70 cm from the transmitter. Second, we recorded the three-dimensional movement of sensors on the anterior and posterior surfaces of the chest wall during maximum voluntary ventilation in four normal men; all sensors were placed in the midsagittal plane of the body. Anterior sensors were located on the sternum at the level of the third intercostal space and at 2 cm above the umbilicus, whereas posterior sensors were located on the posterior spine at the same vertical levels as the anterior sensors. In all subjects the following was found. 1) Both anterior sensors moved anterior and cephalad during inspiration. The anterior thoracic sensor showed greater vertical than anteroposterior (A-P) movement, whereas the anterior abdominal sensor showed greater A-P than vertical movement. 2) Inspiration was associated with spinal extension, whereas expiration was associated with spinal flexion. Third, we used Tri-mag information to 1) measure tidal volume (VT) over a range extending from 500 ml to inspiratory capacity and 2) measure the change in end-expiratory lung volume (EELV) over a range extending from FRC to FRC plus a minimum of 1.5 liters. Our results indicate that greater than 96% of the changes in VT and greater than 82% of the changes in EELV can be accounted for by changes in A-P, vertical, and lateral dimensions of the chest wall.     

Study of the in vitro bioactivation of albendazole in human liver microsomes and hepatoma cell lines

The metabolism of albendazole (ABZ), a benzimidazole anthelminthic, was studied in either microsomal preparations of human liver biopsies or cultured human hepatoma cell lines. Metabolites were analyzed by HPLC. Our data show that microsomes from human biopsies and two human cell lines, HepG2 and Hep3B, oxidize the drug to the sulfoxide very efficiently, whereas the third cell line tested, SK-HEP-1, does not. Both cytochrome P-450 dependent monooxygenases and favin-containing monooxygenases appear to be involved in human ABZ metabolism. Using the cell line displaying the highest ABZ-metabolizing activity, HepG2, the cytotoxic and the inducing effects of the parent drug ABZ and of two primary metabolites, the sulfoxide and the sulfone were studied. These three chemicals provoked a rise in mitotic index resulting from cell division blockage at the prophase or at the metaphase (ABZ metabolites) stage, and ABZ was more cytotoxic than its metabolites. With regard to enzyme-inducing effects, our data clearly demonstrate that the sulfoxide and, to a lesser degree, the sulfone are potent inducers of some drug metabolizing enzymes (i.e., cytochrome P-488 dependent monooxygenases and UDP glucuronyltransferase), whereas ABZ fails to increase and even slightly decreases these enzymatic activities. In conclusion, the HepG2 human hepatoma cell line appears to be suitable for the study of many parameters of metabolism and action of ABZ and other structurally related compounds in humans.Abbreviations ABZ albendazole - B[a]P benzo[a]pyrene - HPLC high-performance liquid chromatography - MC 3-methylcholanthrene - MFO mixed-function oxidase - UDPGT UDP-glucuronyltransferase     

Structure-Biological Activity Relationships in Alfalfa Antimycotic Saponins: The Relative Activity of Medicagenic Acid and Synthetic Derivatives Thereof Against Plant Pathogenic Fungi1

The antimycotic activity of medicagenic acid and of some synthetic derivatives thereof was tested against plant pathogenic fungi. In general they all possess antimycotic activity. Furthermore, in the case of Sclerotium rolfsii, compounds where the hydroxyl functions of the aglycon remained unchanged (medicagenic acid and its dimethyl ester) or could be enzymically released (3-0-β-D-glucoside of medicagenic acid dimethyl ester) were significantly more active than compounds where these functions were modified by acetylation or methylation. Selective 2-0-methylation of medicagenic acid and comparison of the antimycotic activity of the resulting derivative against S. rolfsii to that of other derivatives suggests that a potential free hydroxyl at position 3 is essential to antimycotic activity.     

Stimulation of the respiratory burst in peripheral blood monocytes by lipoteichoic acid. The involvement of calcium ions and phospholipase A2

Lipoteichoic acid (LTA) from Streptococcus faecalis stimulates the respiratory burst in peripheral blood monocytes (mon), as measured by cytochrome C reduction. The effect of LTA was time and dose dependent. LTA stimulated the respiratory burst in a biphasic manner within a range of 1 to 1000 ng/ml.10(6) mon, with maximal activity at 50 ng/ml. At this concentration LTA increased the activity from 0.97 +/- 0.2 to 4.88 +/- 0.2 nmol.10(6) mon/20 min. The role of calcium ions in the effect of LTA in stimulating respiratory burst was studied by changing the availability of calcium ions in the medium, and by measuring the effect of LTA on 45Ca2+ uptake and on intracellular Ca2+ levels. Removal of extracellular calcium ions in the presence of the calcium chelator EGTA, abolished the LTA-stimulated respiratory burst. LTA (50 ng/ml) was found to increase 45Ca2+ uptake into monocytes within seconds (from 2200 +/- 242 in the untreated cells to 4642 +/- 365 cpm/min in the LTA-treated mon). At this concentration, LTA stimulated an immediate rise in the intracellular free Ca2+ concentration to 155 +/- 15 nM as compared with 120 +/- 14 nM in the unstimulated monocytes. LTA caused a specific release of arachidonic acid indicating the involvement of phospholipase A2 in the transduction signal stimulating the respiratory burst by LTA.