Joëlle Noël

In vitro proteolysis of myofibrillar and sarcoplasmic proteins of white muscle of sea bass (Dicentrarchus labrax L.): effects of cathepsins B, D and L

The purpose of this study was to obtain additional information regarding proteolysis mechanisms and disorganization of fish myofibrils resulting in a loss of flesh quality. The ability of cathepsins to degrade in vitro myofibrillar and sarcoplasmic proteins from fish muscle was investigated in order to explain their role in post mortem softening. This led to the identification of substrates of the enzymes. Cathepsins degraded myosin heavy chain and α-actinin. Tropomyosin and actin were only susceptible to the action of cathepsin L. Troponin T (assumed 32 kDa component) was resistant only to the action of cathepsin D. Desmin was degraded by cathepsins B and L. Slight changes of some other myofibrillar or cytosolic proteins were also observed (creatine kinase and other unidentified proteins). When compared with protein modifications observed in stored post mortem muscle, these results suggest that cathepsin D (if location is in the cytosol and if pH conditions for activity are met in post mortem muscle) could be involved in a post mortem myofibrillar degradation mechanism.

Protein changes in post mortem sea bass (Dicentrarchus labrax) muscle monitored by one- and two-dimensional gel electrophoresis

This study was devoted to the identification of specific peptides and proteins which can be used as indicators of freshness in fish. The post mortem evolution of protein patterns in farmed sea bass muscle was monitored by Sodium dodecyl sulfate‐polyacrylamide gel electrophoresis (SDS‐PAGE) and two‐dimensional electrophoresis (2‐DE) after 0, 2, 4, and 6 days cold storage. SDS‐electrophoresis, of total proteins and proteins soluble in low‐ionic‐strength solutions, revealed the gradual disappearance of a protein band of 16 kDa immediately after fish death. 2‐DE allowed the classification of fish samples according to post mortem time. Three spots of interest, which disappeared progressively, were identified on the 2‐DE patterns. Further research is required to establish the identity of these polypeptides and to evaluate their expression and post mortem evolution in another fish species.

Neutral calcium-activated proteases from European sea bass (Dicentrarchus labrax L.) muscle: polymorphism and biochemical studies

Calcium-dependent proteinases or calpains were studied in fish muscle. Hydrophobic chromatography, followed by anion-exchange chromatography of the soluble fraction of sea bass white muscle proteins, resulted in three peaks of calcium-dependent protease activity at neutral pH (A, B and C). They are all neutral cysteine calcium-activated proteinases and can, therefore, be classified as calpain-like enzymes. From the Ca2+ concentration required for activity, A is a mu-calpain, and B and C are m-calpains. They share many properties with calpains from other vertebrate cells but differ in native mass, subunit composition, and the unusual numbers in which they are present. Their specific pattern of expression throughout the year could be of great importance to the resulting rate and extent of degradation of fish flesh after death.

Proteolytic potential in white muscle of sea bass (Dicentrarchus labrax L.) during post mortem storage on ice: time-dependent changes in the activity of the components of the calpain system

The variations in the amounts of milli-calpain and its specific inhibitor in the white muscle of sea bass (Dicentrarchus labrax) during storage at 4 °C for up to 7 days were determined after separation by hydrophobic chromatography on a Phenyl Sepharose gel. There was a significant decline in postslaughter m-calpain activity with an important inter-individual variability in the rate of decrease of the total activity. In contrast with the calpastatin of mammalian post mortem muscles, calpastatin remained constant within fish muscles after death. The initial levels of protease and inhibitor activities, and their behaviour through post mortem storage, are discussed and implications for the mechanism of tenderisation of fish muscle are suggested.

Mercury transport in waters of the strait of dover

The purpose of this work as part of the FluxManche EC Project (MAST-1) was to obtain accurate data on the concentrations and distributions of dissolved and particulate mercury which, together with determinations of suspended matter and water transport, can provide estimates of fluxes through the Strait of Dover. Samples were collected every other month between September 1990 and December 1991 at six stations, two depths being sampled at each station, on a transect between Folkestone and Cap Griz-Nez. Data concentrations were log-normal distributed. Geometric means were 2.65 and 3.36 pM for dissolved and particulate mercury, respectively. Mercury linked to particles constituted between 20 and 97% of total mercury, with the highest proportions being found near the coast similarly to the distribution pattern of particulate matter along the transect. On the basis of water and suspended particulate matter transports, estimated mercury fluxes from the English Channel to the North Sea were 1.8 and 2.3 t a−1 for dissolved and particulate mercury, respectively.