Altered acetylcholinesterase isozyme patterns in mice with hereditary muscular dystrophy

Acetylcholinesterase inhibition by 1-methyl-4-phenylpyridinium ion, a bioactivated metabolite of MPTP

The effect of the neurotoxicant, 1-methyl-4-phenylpyridinium ion (MPP+) on acetylcholinesterase (AchE) activity was investigated. The MPP+ was found to inactivate the enzyme in a dose dependent manner. The kinetic parameter, Km for the substrate (acetylthiocholine), was found to be 0.216 mM and Ki for MPP+ for the inactivation of AChE was found to be 0.197 mM. It was found that MPP+ is neither a substrate of AChE nor the time-dependent inactivator. The studies of reaction kinetics indicate the inactivation of AChE to be a linear mixed-type inhibition. The inactivation of AChE by MPP+ was partially recovered by either dilution or gel exclusion chromatography. These data suggest that once MPP+ enters the basal ganglia of the brain, it can inactivate the AChE and thereby increase the acetylcholine level in the basal ganglia, leading to potential cell dysfunction. It appears likely that the nigrostriatal toxicity by MPP+ leading to Parkinson's disease-like syndrome may, in part, be mediated via the AChE inactivation.

Fiber-specific cholesterol changes in murine dystrophy

The cholesterol concentration in dystrophic mouse muscle is reported to be increased compared to normal. The muscles studied are, however, composed in most cases of more than one fiber type. As a result, the observed concentration increase may be due to a general increase or may be due to changes in the proportion of individual fiber types which themselves differ in cholesterol concentration. To decide between these possibilities we have measured the cholesterol concentrations (both free cholesterol and cholesterol esters) in normal and dystrophic whole gastrocnemius muscles and compared the values with the concentrations in fast-glycolytic muscle tissue alone. The cholesterol concentrations in both whole and fast-glycolytic sections of dystrophic muscle are increased compared to normal, with the largest increase in the cholesterol ester fraction. Furthermore, the concentration changes in fast-glycolytic fibers are due mainly to cholesterol ester differences in both membrane and sarcoplasm fractions, with differences in the latter being larger. The data show that changes in whole muscle concentrations cannot be ascribed solely to altered fiber type proportions.

Inactivation of rat brain acetylcholinesterase by pyridoxal 5'-phosphate

Effects of pyridoxal 5'-phosphate on the activity of crude and purified acetylcholinesterase from cerebral hemispheres of adult rat brain were examined. Acetylcholinesterase was completely inactivated by incubation with 0.5 mM pyridoxal 5'-phosphate. The enzyme activity remained unaltered in the presence of analogs of pyridoxal 5'-phosphate, pyridoxal, pyridoxamine and pyridoxamine 5'-phosphate. The inhibition of acetylcholinesterase activity by pyridoxal 5'-phosphate appeared to be of a noncompetitive nature, as determined by Lineweaver-Burk analysis. The inhibitory effect of pyridoxal 5'-phosphate on acetylcholinesterase appeared to be a general one, as the activity of the enzyme from the brains of immature chick and egg-laying hen, and from different tissues of the adult male rats, exhibited a similar pattern in the presence of the inhibitor. The inhibitory effects of pyridoxal 5'-phosphate could be reversed upon exhaustive dialysis of the pyridoxal 5'-phosphate-treated acetylcholinesterase preparations. We propose that the effects of pyridoxal 5'-phosphate are due to its interaction with acetylcholinesterase, and that it can be employed as a useful tool for studying biochemical aspects of this important brain enzyme.

The acetylcholinesterase abnormality in dystrophic mice is a reflection of a maturational defect

Juvenile mice of the ReJ/129 strain exhibit a distribution of acetylcholinesterase molecular forms that is similar to the pattern previously observed in dystrophic mice. The change to an adult distribution of the enzyme occurs at about 3 weeks of age which is also when dystrophic signs first become evident. It is suggested that dystrophic mouse muscle fails to mature with respect to the molecular forms of acetylcholinesterase.

Asymmetric and globular forms of AChE in slow and fast muscles of 129/ReJ normal and dystrophic mice

Acetylcholinesterase activities and molecular forms were studied in normal and dystrophic 129/ReJ mice, focusing on four predominantly fast-twitch muscles and the slow-twitch soleus. The asymmetric and globular forms were analyzed separately so that the effect of dystrophy on each form could be determined. This comparative study showed the following. (1) In the normal condition, each muscle exhibited a distinct distribution of the molecular forms. (2) The diversity among the fast muscles resulted mainly from variations in the proportions of the three globular forms; in contrast, these muscles showed a constant and precise A12/A8/A4 ratio. (3) The slow-twitch soleus clearly differed from the other muscles in its low acetylcholinesterase activity and distinct distribution of the molecular forms, characterized by a low level of G4 and a peculiar ratio among its asymmetric forms, resulting from a relative increase of the A8 and A4 forms. (4) In dystrophic mice, the diversity of the acetylcholinesterase distribution was lost; all the fast muscles displayed profiles exhibiting the characteristics typical of the soleus. The fast-twitch extensor digitorum longus, sternomastoid, and plantaris converged towards an identical set of acetylcholinesterase molecules. (5) In contrast, the acetylcholinesterase activity and molecular forms of the soleus were only slightly affected by the disease. These results reveal that the dystrophy modifies both categories of molecular forms of acetylcholinesterase in a very precise manner. Such complex changes, which are highly reproducible in a variety of different muscles, are unlikely to result from nonspecific reactions secondary to the disease.