Variation in cyanobacterial hepatotoxin (microcystin) content of water samples and two species of fishes collected from a shallow lake in Algeria.

Microcystins (MCs) produced from cyanobacteria can accumulate in freshwater fish tissues. In this study, variations in these toxins content were examined monthly in water samples and two species of fish in Lake Oubeira, Algeria, from April 2010 to March 2011. During the study period, MCs were analyzed using protein phosphatase type 2A (PP2A) inhibition assay. In lake water, total (dissolved and intracellular toxins) MC concentrations by PP2A ranged from 0.028 to 13.4 µg equivalent MC-LR/l, with a peak in September 2010. MC-LR was the dominant variant (90 % of the total) in water samples, followed by MC-YR and MC-(H4)YR. The highest MC concentration in the omnivorous common carp (Cyprinus carpio) was found in the order intestine > hepatopancreas > muscle; however, in the carnivorous European eel (Anguilla anguilla) the order was liver > intestine > muscle. Highest MC concentrations in the intestine tissue of the common carp were found between August and November 2010 where high MC concentrations were detected in water samples, whereas high levels of MCs in the liver of the European eel were found later between January and February 2011. During the entire period of study, the World Health Organization (WHO) lifetime limit for tolerable daily intake was exceeded only in common carp muscle.

Evidence that thermodynamic stability of papaya glutamine cyclase is only marginal.

Papaya glutamine cyclase (PQC), a glycoprotein with a molecular mass of 32,980 Da, is a minor constituent of the papaya latex protein fraction. In neutral aqueous solutions, PQC adopts an all-beta conformation and exhibits high resistance to both proteolysis and denaturation. Complete unfolding of PQC requires a combination of an acidic medium and chemical denaturant such as urea or guanidine hydrochloride. The unfolding process takes place through formation of an intermediate A state that accumulates in the absence of chemical denaturants and displays all the features of a molten globule state. The different conformational states-N (native), A (acid-inactivated), and U (unfolded)-have been characterized by means of circular dichroism measurements, fluorescence spectroscopies, Stokes radii determinations, and 8-anilino-1-naphtalenesulfonic acid (ANS) binding characteristics. The unfolding pathways of the enzyme was further studied to estimate thermodynamic parameters characterizing both transitions N if A and A if U. In its A state, PQC is catalytically inefficient and highly susceptible to proteolysis. Also, its thermodynamic stability is decreased by some 3-5 kcal/mol. Conversion of the native to the A state involves digging up of five amino functions together with protonation of four to five acidic groups with pK(a)s, in the native state, around 2.7. It proceeds both cooperatively and reversibly although, in vitro, the refolding process is slow. Unfolding of the A state, on the other hand, occurs with a low degree of cooperativity. The intermediate A state thus seems to be only marginally more stable than the unfolded state. The role of suspected internal ion pairs in the stabilization of the native state of this enzyme is discussed.