The effect of mercury and aluminum on sodium-potassium-Mg2+ dependent-adenosine triphosphatase activity of Electrophorus electricus (L.) electrocyte

The cytoskeleton of the electric tissue of Electrophorus electricus, L

The electric eel Electrophorus electricus is a fresh water teleost showing an electrogenic tissue that produces electric discharges. This electrogenic tissue is distributed in three well-defined electric organs which may be found symmetrically along both sides of the eel. These electric organs develop from muscle and exhibit several biochemical properties and morphological features of the muscle sarcolema. This review examines the contribution of the cytoskeletal meshwork to the maintenance of the polarized organization of the electrocyte, the cell that contains all electric properties of each electric organ. The cytoskeletal filaments display an important role in the establishment and maintenance of the highly specialized membrane model system of the electrocyte. As a muscular tissue, these electric organs expresses actin and desmin. The studies that characterized these cytoskeletal proteins and their implications on the electrophysiology of the electric tissues are revisited.

Electrophorus electricus as a model system for the study of membrane excitability

The stunning sensations produced by electric fish, particularly the electric eel, Electrophorus electricus, have fascinated scientists for centuries. Within the last 50 years, however, electric cells of Electrophorus have provided a unique model system that is both specialized and appropriate for the study of excitable cell membrane electrophysiology and biochemistry. Electric tissue generates whole animal electrical discharges by means of membrane potentials that are remarkably similar to those of mammalian neurons, myocytes and secretory cells. Electrocytes express ion channels, ATPases and signal transduction proteins common to these other excitable cells. Action potentials of electrocytes represent the specialized end function of electric tissue whereas other excitable cells use membrane potential changes to trigger sophisticated cellular processes, such as myofilament cross-bridging for contraction, or exocytosis for secretion. Because electric tissue lacks these functions and the proteins associated with them, it provides a highly specialized membrane model system. This review examines the basic mechanisms involved in the generation of the electrical discharge of the electric eel and the membrane proteins involved. The valuable contributions that electric tissue continues to make toward the understanding of excitable cell physiology and biochemistry are summarized, particularly those studies using electrocytes as a model system for the study of the regulation of membrane excitability by second messengers and signal transduction pathways.

Comparison of the inhibitory effects of mercury and cadmium on the creatine kinase from Electrophorus electricus (L)

We have determined the effects of mercury and cadmium on the creatine kinase activity of the electric organ of Electrophorus electricus (L.) which catalyses the transphosphorylation reaction between phosphocreatine and magnesium adenosine-5'-di-phosphate and has essential sulfhydryl groups. The kinetic effects of these heavy metals, which have high affinity for sulfhydryl groups, on the creatine kinase activity were analysed with the three reaction components: phosphocreatine, adenosine-5'-di-phosphate and magnesium. The kinetic data were analysed with a non-linear regression program (Sigmaplot for Windows). Both metals inhibit creatine kinase activity in the micromolar range, mercury being a more potent inhibitor than cadmium. With phosphocreatine as substrate, mercury behaved as a mixed partial hyperbolic inhibitor, non-competitive inhibitor with adenosine-5'-di-phosphate, and with magnesium mercury behaved as a competitive inhibitor. Cadmium inhibition was shown to be of a classical competitive nature with respect to both substrates, phosphocreatine or adenosine-5'-di-phosphate, and non-competitive when magnesium was the variable in the reaction mixture. The results suggest that the binding site of mercury is at or near the phosphocreatine site, but it is not the same as adenosine-5'-di-phosphate, whereas cadmium competes with these substrates to bind at the same sulphydryl site.

Different effects of aluminum upon carbonic anhydrases and Na+/K+-ATPase activities in rat

Aluminum (Al(III)) is a well established toxicant implicated as an etiological factor in several neuropathies. In this paper we report results regarding opposite effects produced by Al(III) on the activity of two enzymes utilized as models. While sodium-potassium ATP-ase (Na/K-ATPase) is strongly activated by Al(III) in a dose-effect dependent way, on the contrary, carbonic anhydrase (CA) is remarkably inhibited. The relevance of the metal speciation together with the enzymatic structural modification demonstrated by circular dichroism measurements could explain the observed modified enzymatic activities. In addition, a new experimental protocol for the preparation of Al(III) solutions at physiological pH useful in the standardization of Al(III) experimental toxicology is also proposed and discussed.