Structure and function of the calcium pump protein of sarcoplasmic reticulum

Recent developments concerning the structure and function of the Ca2+ pump protein of the sarcoplasmic reticulum have been briefly reviewed. Various new methods have become available that make it possible to monitor dynamic changes in the structure of the enzyme molecule associated with elementary steps of the enzyme reaction. In the light of information about chemical reactivity of various amino acid residues and their location in the primary structure of the ATPase polypeptide, it will be fruitful to use extrinsic conformational probes placed at specific locations to monitor the kinetics of the enzyme. Furthermore, a growing body of evidence suggests that subunit-subunit interactions of an oligomeric Ca2+ ATPase are involved in the regulation of the kinetics of the enzyme. Thus the kinetic mechanisms has to be reinterpreted at all levels--i.e. primary, secondary, tertiary, and quaternary--of structure.

Role of cysteine in regulating morphogenesis and mitochondrial activity in the dimorphic fungus Histoplasma capsulatum

Three stages can be distinguished in the temperature-induced mycelial-to-yeast phase transition of Histoplasma capsulatum. Stage one is characterized by a progressive decrease in the respiration rate and in the intracellular concentrations of cysteine and other amino acids. By stage two, respiration has ceased completely and free cysteine has fallen to low levels. Exogenous cysteine is required during the second stage for activation of mitochondrial respiration (stage three) and completion of the morphological transition. Mitochondria isolated from cells in the second stage show no respiration with NADH, succinate, or other substrates unless they are first incubated with cysteine. In addition, a novel, cytosolic cysteine oxidase appears during the latter part of the second stage. In stage three, the respiration rate rises, intracellular concentrations of free cysteine and other amino acids increase to levels characteristic of yeast, and the morphological transition is completed. The results support the idea that alterations in cysteine metabolism play a key role in this differentiation process.

Effects of disulfide bond reduction on the excitatory and inhibitory postsynaptic responses of Aplysia ganglion cells

Three kinds of the cholinoceptive neurons, nicotinic depolarizing (D)-, nicotinic hyperpolarizing (H)-, and muscarinic H-tyes, as well as two other kinds of neurons, GABA H- and dopamine H-types, were identified in Aplysia abdominal ganglion, and the effects of disulfide bond reduction and reoxidation on their postsynape acetylcholine-induced responses of both nicotinic types (D- and H-) were depressed by reducing the disulfide bonds with dithiothreitol (DTT) and restored by reoxidizing with 5, 5' -dithiobis-(2-nitrobenzoic acid): (DTNB), whereas the responses of the muscarinic H-, GABA H-, and dopamine H-cells were not affected at all by either DTT or DTNB. In contrast to the results obtained from the electroplax, the cholinergic receptors in our preparation showed neither the activation by hexamethonium nor the augmentation of decamethonium-induced responses after reduction of disulfide bonds. In addition, our preparation did not demonstrate the long-lasting responses to bromoaTT-induced depression of the nicotinic responses was studied on the dose-response curves; the mode of receptor inhibition was rather complexed, being neither type of competitive nor non-competitive. We concluded that the disulfide bond is a crucial element in both types of nicotinic receptors (D and H), and that this bond is related to the activation process of the receptors regardless of their ionic specificities.