Enzymological investigations on the causes for the PSE-syndrome, II. Comparative studies on glycogen phosphorylase from pig muscles

In order to investigate the cell biological causes for the fast breakdown of glycogen which is observed during the development of the PSE (pale, soft, exudative) syndrome in muscles of stress-susceptible pigs, muscle glycogen phosphorylase (GP) as a key enzyme in two isoforms, a and b, of the energy turnover was isolated from M. longissimus dorsi of normal and PSE-prone pigs of the German Landrace. GP b as well as GP a from normal and PSE-muscles exist in a dimeric form with a molecular weight of 97 000 D per subunit. The tendency for tetramerization of GP b increases in the presence of ATP, whereas the enzyme activity is simultaneously inhibited. The catalytic activities of GP a and GP b from both groups of animals show an optimum at pH 7.0. GP b can be activated to GP a by phosphorylation with the result of a 25% higher optimum specific activity in the case of normal and PSE-muscles. In interaction with glycogen and glucose-1-phosphate GP b follows the characteristics of a Michaelis-Menten kinetic, whereas the binding of AMP and phosphate proves to be allosteric. In comparison of the structural and kinetic characteristics of GP from normal as well as PSE-muscles no significant differences could be determined, indicating that GP does not belong to those factors which are triggering an accelerated energy turnover of ATP in muscles of stress-susceptible pigs.

[Quality of ecologically produced foods of animal origin]

The production of organic (ecological) food of animal origin is done in many ways and uses many different breeds. Therefore a real comparison with conventionally produced food is difficult. From the limited number of published data it appears, that the characteristics of quality of the products, the nutritional, hygienic, sensorial and technological factors, are not very different in both systems of production. In some factors organic food gets better marks, in others the conventionally produced food. The differences are in the production system (process quality) during lifetime of the animals.

Comparative studies of pyruvate kinase from PSE and normal pig muscles

A fast breakdown of glycogen is observed in muscles of stress-susceptible pigs leading to pale, soft and exudative (PSE) meat. We report a comparative study of pyruvate kinase from muscles of normal and PSE-prone pigs. Compared with the enzyme from normal muscle, pyruvate kinase isolated from PSE muscle shows a five times lower Michaelis constant, Km, for phosphoenol pyruvate and a more than ten times higher Kcat/Km value. The pH dependency of the enzymatic activity is shifted to more acidic values for pyruvate kinase from PSE muscles. According to isoelectric focusing, pyruvate kinase from PSE muscle consists of three isoforms, while only two isoforms are detectable in pyruvate kinase preparations from normal pigs. The various isoforms were isolated by preparative isoelectric focusing and their steady-state properties were compared. Isoform 3, which is found only in PSE muscle, shows a 10-fold higher specific activity, a 30-fold lower Km value and a 100-fold increased kcat/Km value for phosphoenol pyruvate as compared to isoform 1. The presence of isoform 3 in PSE muscle appears to be responsible for the high activity of this enzyme under the more acidic conditions prevailing in PSE muscle. In vitro phosphorylation and dephosphorylation experiments using total enzyme and purified isoenzyme 1 suggest that isoforms 2 and 3 arise from isoform 1 by phosphorylation. Thus protein phosphorylation seems to be responsible for the shift in activity of pyruvate kinase, a key enzyme of glycolysis, under the acidic conditions of PSE muscles.

Assessment of data sets containing a considerable number of values below the detection limits

Using real examples it is demonstrated that the concentrations of non-essential substances in biological systems always follow a statistical log-normal distribution. This is based on the assumption that in a first-order approximation the rate of excretion of the quantity of unwanted molecules or atoms in the biological system is only dependent on their concentration. Applying a natural logarithmic transformation, the resulting log-normal distribution always has a standard deviation of about 1, as shown by statistical analysis of more than 680 clusters of analytical results. Assuming a log-normal distribution with a standard deviation of 1, it is possible to derive factors for the estimation of percentiles, such as the median, even when this percentile is below the detection limit; quantifiable and higher percentiles only must be multiplied by a specific factor. This new method has the advantage that no arbitrary assumptions must be made about artificial parameters such as the numerical treatment of non-detectable analytical data. The assumptions used for the calculation of the factors are justified by proof with experimental data. The applicability of the new method is demonstrated for various contaminants in various plants and animals.

Purification and isoenzymic composition of glycogen phosphorylase b from normal and abnormal (PSE) muscles

Ion exchange chromatography and preparative isoelectric focusing allowed the identification of five isoenzymes of glycogen phosphorylase b from the longissimus dorsi muscle of normal pigs and those prone to having pale, soft and exudative (PSE) muscle. The isoelectric point of the isoenzymes varied in the pH range from 6.29 to 6.55. One of them, with an isoelectric point at about a pH of 6.49, accounts for 65% of the total glycogen phosphorylase b activity. No significant differences between normal and PSE-prone pigs were observed in the total glycogen phosphorylase b activity and in the isoenzyme distribution pattern. It is concluded that the fast glycogen turnover in PSE-prone pigs is not due to a different isoenzyme pattern of phosphorylase b.

Changes in histological structure and physiochemical properties of raw, cured, smoked pork loins resulting from freezing near cryoscopic temperature

The aim of the study was to determine the effects of unintended freezing during cold storage (deep chilling conditions) and low freezing temperatures (-10 degrees C) of raw pork loins, cured and smoked, on their histological structure and some physicochemical characteristics. The experimental products after freezing at -10 degrees C, were stored at the same temperature for 0, 2, 4 and 6 days, thawed thereafter and immediately deep chilled at near cryoscopic temperature (-3 degrees C) for 0, 2, 4 or 6 days. These conditions were changing the histological structures of pork loin, the water holding capacity (WHC) was reduced by frozen storage but improved by deep chilling; the maximum shear force and energy were increased. The more the freezing at -10 degrees C was prolonged, the more unfavourable were the changes in structure, WHC and tenderness. Deep chilling at -3 degrees C after thawing of the experimental material on the other hand favourably affected the parameters measured. Freezing at -10 degrees C and deep chilling at -3 degrees C did not affect the electrophoretic picture of pork loin proteins nor the water activity. Freezing itself had no influence on WHC. Changes were due to storage.

The influence of mincing and temperature of storage on the oxidation of pork back fat and its effect on water- and fat-binding in finely comminuted batters

Besides the ability of swollen and dissolved meat proteins to form a heatstable network in finely comminuted sausage batters, the fat in the batter is an additional factor of batter stability. Fat cell integrity and the melting point of fat are regarded in the literature as essential for fat retention. In practice, however, butchers report that slaughterfresh pork back fat, whilst within a few hours post mortem and still hot, enhances batter stability compared with chilled and stored back fat. Experiments confirmed this observation. There could be several reasons for this behaviour. One would be the oxidation processes in pork fat which occur on storage. The experiments reported here show that rancidity of fat has little or no influence on batter stability. Disintegration of fat cells by mincing and/or fat softness by increased temperature, enhance batter stability. The conclusion drawn emphasizes the importance of the early formation of small fat particles in the batter for batter stability.

[Myoglobin content and beta-hydroxyacyl-CoA-dehydrogenase activity of different muscles of cattle and calf]

The heart, tongue, jowl, diaphragm and tail as well as shoulder, top round, the longissimus dorsi muscle of slaughtered cattle and the diaphragms of calf were examined with respect to their myoglobin content and beta-hydroxyacyl-CoA-dehydrogenase (HADH) activity. According to Gottesmann and Hamm [1] the product of these two values, the so-called MH value, can serve as the differentiation between the diaphragm and "normal cross striated skeletal muscles". Like the diaphragm, heart, tongue and jowl of cattle show higher MH values than those of "normal beef". Muscles in the tail have the same MH values as those of normal beef muscles. There are no essential differences in the MH values of various cross-striated muscle types of cows and calves. Muscles of cattle show a slightly higher myoglobin content, whereas the HADH activity is lower than in veal.

[Influence of below-freezing temperatures on the rate of post-mortem metabolism and the water-holding capacity in prerigor frozen beef muscles (author's transl)]

Prerigor beef is very suitable for the production of "bruehwurst" (frankfurter and bologna type sausages) because of its high waterholding capacity (WHC). This high WHC can be preserved for several months by rapid freezing of prerigor beef either unsalted or salted. In order to elucidate the optimum conditions of frozen storage for the preservation of high WHC, intact beef muscle, ground beef and ground, salted (2% NaC1) beef (neck muscles) were frozen at - 18 degrees or --40 degrees C in a thin layer (0.5-1 cm) 30--60 min after slaugther and stored at various temperatures between --5 degrees and --40 degrees C. The changes of biochemical parameters (content of glycogen and lactate, R-value [it corresponds to ATP concentration] and pH) in the tissue and the WHC of raw and heated muscle homogenates, prepared from the frozen material and containing 2% NaC1, were measured and related to time and temperature of frozen storage. At storage temperature of --18 degrees and --40 degrees C no appreciable biochemical changes occur in intact and ground, unsalted tissue over a period of 10 months. Above 18 degrees C, however, rising temperature causes an increased rate of ATP turnover and glycolysis. In ground muscle higher rates of postmortem metabolism are generally found than in the intact tissue. In prerigor salted and frozen beef a faster drug of ATP concentration (increase of R-value) occurs whereas the breakdown of glycogen to lactate is inhibited. The WHC of raw and heated muscle homogenates depends on time and temperature of frozen storage. As soon as the ATP concentration in unsalted beef falls to a level, at which the onset of rigor mortis occurs, the WHC of homogenates decreases markedly. With beef, ground and salted in the prerigor state and than frozen muscle homogenates are obtained which show always a high WHC even after complete breakdown at ATP during frozen storage. These results have practical consequences with regard to processing of hot-deboned beef.

Effect of storage and processing on tetracycline residues in meat and bones

A semiquantitative microbiological screening test for antibiotics, a sensitive and quantitative microbiological assay, and a fluorometric method specific for tetracyclines are described. Using these procedures, tetracycline residues in animals derived from feed can be detected in tissues like organs, muscles, and bones. Meat contaminated with chlortetracycline (CTC) and oxytetracycline (OTC) and stored at +8 degrees C and -22 degrees C showed very little decrease in antibiotic concentration; however, heating above 65 degrees C reduced the tetracycline content in meat. Temperatures above 130 degrees C were necessary to destroy CTC in bones, CTC in bones was insoluble above pH 4. Manufacturing products with contaminated meat reduced the tetracycline content only if heating was involved.

[Fast fluorimetric assay of chlortetracycline in meat and bone (author's transl)]

Antibiotics in biological materials, including tetracyclines, are determined usually be means of microbiological methods. Because of the low specificity and the time consuming procedure of the microbiological assays, we developed a fluorimetric method, which enables us to determine specifically the tetracyclines and to reduce the time for the assay down to 4 hours. Our method is based on the determination of tetracyclines published by Kohn (1961), which we modified for the determination of chlortetracycline in meat and bone. Tetracyclines are fluorescent compounds, whose quantum yields are very much enhanced in most cases by forming a complex with Ca2+ ions and by extracting it into an organic solvent. This transfer into acetic acid ethylester separates the chlortetracycline from interfering substances. The yield of chlortetracycline with the described method was about 80%, the sensitivity of the detection was about 0.5 ppm.

Studies on allosteric phenomena in glycogen phosphorylase b

This article attempts to trace, from a personal point of view, the history of discoveries of allosteric phenomena in phosphorylase b and the later development of systematic attempts to fit the data into comprehensive theoretical models. Work from our own laboratory is emphasized, but we try to integrate this into the results from other investigators and show their contributions to our ideas and experiments. Finally, some recent unpublished data is presented together with some conclusions and predictions from a new hypothesis. The discoveries by Carl and Gerty Cori of the activation of phosphorylase by AMP, the inhibition of glucose and the enzymatic interconversion of two forms fo the enzyme with different control properties helped lay the foundations of our present understanding of allosteric mechanisms. The later discovery of the oligomeric nature of phosphorylase and its relationship to AMP binding served as a basis for many years of research into the structure-function relationships of phosphorylase and other enzymes. Data showing that AMP lowers the entropy of activation is discussed with respect to the role of the nucleotide and its binding close to the active site. The discovery of the control of phosphorylase b by common metabolites and the impetus this gave to the intensive kinetic studies of the last ten years, wherein fitting to theoretical models has been a common feature, is reviewed.

Fluorescence quenching, a tool for probing conformational changes in glycogen phosphorylase

This study shows that conformational changes in glycogen phosphorylase are accompanied by changes in the accessibility of tryptophan residues and of the coenzyme, pyridoxal phosphate, to the surrounding aqueous medium. The accessibility was estimated by determining the extent to which iodide can quench the fluorescence emission of these moieties by colliding with them, since iodide cannot collide with a buried chromophore and hence cannot quench its fluorescence. Rabbit muscle phosphorylase b, its apoform, and phosphorylase a exhibit differences in the number of exposed tryptophans, while the phosphorylase b forms from rabbit skeletal muscle and pig heart also show differences.Differences are also observed in the accessibility of the coenzyme in different forms of the enzyme. The quenching rate constant, a measure of accessibility, differs for phosphorylases a and b, and this constant is affected differently by ligand binding to the two forms. While the allosteric inhibitors, ATP and glucose 6-phosphate, render the pyridoxal phosphate moiety of phosphorylase b more accessible, the activator, AMP, and substrate, glucose 1-phosphate, together cause it to be totally inaccessible to fluorescence quenching by iodide. AMP and glucose 1-phosphate appear to mediate a conformational change which buries the coenzyme. While pyridoxal phosphate is necessary for catalytic activity, one may conclude from these experiments that its ring structure is unlikely to participate directly in the catalytic mechanism.

Relationship between catalytic activity, crystalline state, and environment of the coenzyme in phosphorylase a

Muscle glycogen phosphorylase a contains a small proportion of its coenzyme, pyridoxal phosphate, bound in a Schiff base form which absorbs at 415 nm. Excited at this wavelength, it fluoresces with two maxima, 510 nm and 535–540 nm. The ratio of the latter peak height to that of 510 nm varies directly with the specific catalytic activity. Experiments on phosphorylase a with increasing concentrations of imidazole citrate show very similar results. When interpreted in analogy to solvent perturbation effects on model compounds, this finding suggests that the catalytic efficiency depends on the degree of hydrophobicity of the pyridoxal phosphate environment, and provides one more link between the coenzyme and the catalytic process.A single large crystal of phosphorylase a, when excited at 330 nm, exhibits nearly half of its fluorescence emission at 415 nm and the remainder at 510–535 nm, whereas in solution only a small amount of fluorescence is seen at 415 nm. Furthermore, decreasing the temperature of the crystal increases the proportion of fluorescence emission at 415 nm. These results are interpreted on the basis of the hypothesis that the coenzyme is bound in the form of a carbinol amine, absorbing at 330 nm, which is split on excitation to form a Schiff base which emits as such at 535 nm. The crystal structure of the protein requires more energy to effect the transition to the Schiff base, so that a large proportion of the coenzyme molecules fluoresce as the carbinol amine form, while decreasing the temperature increases this proportion even further.