The interaction of auramine O with calmodulin: location of the binding site on the connecting strand.

The cationic dye auramine O forms a fluorescent complex with Ca(2+)-liganded calmodulin. One moderately strong binding site is present, as well as one or more weaker sites. The binding site for auramine O is different from those for toluidinyl-naphthalene sulfonate. The dependence of binding upon electrolyte concentration suggests a substantial electrostatic component of the free energy of binding. The splitting of the bond between residues 77 and 78 by trypsin digestion abolishes auramine O binding; the N- and C-terminal half-molecules have virtually no binding capacity. This suggests that the primary binding site is located near the midpoint of the connecting strand and includes elements of both half-molecules. Thrombin digestion, which splits calmodulin between residues 106 and 107, also substantially reduces auramine O binding; this may be interpreted in terms of the stabilization of the structure of the connecting strand by interaction with residues within binding domain IV. The binding affinity at pH 5.0, where the helical organization of the connecting strand may be intact, is greater than at neutral pH.     

Oil secretion and the pollination of Colpias mollis (Scrophulariaceae)

 Colpias mollis is a perennial lithophyte that secretes non-volatile oil as a pollinator reward. Its white to yellow flowers bear two narrow pouches lined with glandular oil-secreting trichomes. The pollinating bee, Rediviva albifasciata (Melittidae), collects oil with its forelegs and midlegs by rubbing them against the gland patches within the flower. The presence of specialised hairs on these legs facilitates oil-collection. The strongly congruent geographic distributions of C. mollis and R. albifasciata and their close morphological fit suggest a long and close association between plant and pollinator. Although C. mollis appears dependent on R. albifasciata for pollination, R. albifasciata is not always dependent on C. mollis for oil. It can also obtain and utilise oil from flowers of the annual herb Hemimeris racemosa (Scrophulariaceae). At the main study site, fruit set of C. mollis was high (75.7%; N=202 flowers) in a dry year when H. racemosa was rare and relatively low (11.4%; N=184 flowers) in a wet year when H. racemosa was common. Received July 15, 2001; accepted June 17, 2002 Published online: November 20, 2002 Addresses of the authors: Kim E. Steiner (e-mail: ksteiner@calacademy.org), Compton Herbarium, National Botanical Institute, Kirstenbosch, South Africs; Current address: Department of Botany, Califormia Academy of Sciences, Golden Gate Park, San Francisco, CA 94118, USA. V. B. Whitehead, South African Museum, P.O. Box 61, Cape Town 8000, South Africa; Current address: 87 Bruke Street, Graaff-Reinet 6280, South Africa.     

The role of phosphorylation in the α-adrenergic-mediated inhibition of rat hepatic pyruvate kinase

Phenylephrine in the presence of 1-methyl-3-isobutylxanthine and propanolol caused a 40–50% inhibition of pyruvate kinase (type L) activity in isolated hepatocytes, which was accompanied by a 2–3-fold increase in the phosphate content of the enzyme. These changes were blocked by the α-adrenergic antagonist dihydroergocryptine and could not be accounted for by the slight increase in cyclic AMP-dependent protein kinase activity generated by the α-adrenergic agonist. It is concluded that a significant component of the inhibition of hepatic pyruvate kinase mediated by α-adrenergic agonists can be attributed to a cyclic AMP-independent alteration in the phosphorylation state of the enzyme.     

Dopamine-beta-hydroxylase activity in plasma obtained from the pulmonary artery and left ventricle of man

Dopamine-β-hydroxylase activity (DBH) has been measured in plasma obtained simultaneously from the pulmonary artery and left ventricle of fourteen patients who underwent diagnostic cardiac catheterisation. In the majority of these subjects the levels of enzyme activity in the arterial and venous blood were similar, indicating that inactivation of DBH had not occurred in its passage through the pulmonary circulation. One patient with pulmonary hypertension had a large a-v difference in enzyme activity that may have been caused by altered pulmonary haemodynamics. Three of the five subjects that undertook a standardised exercise test on a bicycle ergometer showed a significant increase in plasma DBH activity. There was no correlation between the increases in DBH activity and in cardiac index and heart rate.