Mitochondrial treason: a driver of pH decline rate in post-mortem muscle?

A single entity, the AMP-activated protein kinase (AMPK), phosphorylates and regulates in vivo hydroxymethylglutaryl-CoA reductase and acetyl-CoA carboxylase (key regulatory enzymes of sterol synthesis and fatty acid synthesis, respectively), and probably many additional targets. The kinase is activated by high AMP and low ATP via a complex mechanism, which involves allosteric regulation, promotion of phosphorylation by an upstream protein kinase (AMPK kinase), and inhibition of dephosphorylation. This protein-kinase cascade represents a sensitive system, which is activated by cellular stresses that deplete ATP, and thus acts like a cellular fuel gauge. Our central hypothesis is that, when it detects a 'low-fuel' situation, it protects the cell by switching off ATP-consuming pathways (e.g. fatty acid synthesis and sterol synthesis) and switching on alternative pathways for ATP generation (e.g. fatty acid oxidation). Native AMP-activated protein kinase is a heterotrimer consisting of a catalytic alpha subunit, and beta and gamma subunits, which are also essential for activity. All three subunits have homologues in budding yeast, which are components of the SNF1 protein-kinase complex. SNF1 is activated by glucose starvation (which in yeast leads to ATP depletion) and genetic studies have shown that it is involved in derepression of glucose-repressed genes. This raises the intriguing possibility that AMPK may regulate gene expression in mammals. AMPK/SNF1 homologues are found in higher plants, and this protein-kinase cascade appears to be an ancient system which evolved to protect cells against the effects of nutritional or environmental stress.

The aim of the experiment was to determine the technological properties of meat from Hampshire pigs, as compared to good quality meat from Large White pigs and to exudative meat from halothane-positive (HP) Pietrain pigs. All Hampshire and Large White pigs were halothanenegative (HN). In 129 females and castrated males (47 Large White, 20 HN Pietrain, 27 HP Pietrain, 35 Hampshire), several quality characteristics were measured on raw meat and one ham was processed into cooked 'Paris ham'. Although the pH was normal I h post mortem in Hampshire pigs (which, in this respect, did not differ from Large White pigs) pork from Hampshire pigs, especially females, showed a very low ultimate pH and the highest cooking loss in processing. However, meat was much less exudative when fresh and generally darker (reflectance measured at 630 nm or subjective colour score) in Hampshire than in HP Pietrain pigs. A low ultimate pH occurred in muscle from Hampshire pigs owing to a very high 'glycolytic potential' (essentially glycogen content). It is proposed to use the term 'Hampshire type' to denote meat whose qualitative inadequacies basically result from an abnormally lowered ultimate pH, and to keep the term 'PSE' to refer to meat whose exudative state comes from an abnormally rapid pH fall in the immediate postmortem period. Copyright © 1985. Published by Elsevier Ltd.