Competition during innervation of embryonic amphibian head skin

We have examined the initial innervation of the head skin in Xenopus laevis embryos which is by two classes of trigeminal mechanoreceptor with beaded 'free' nerve-endings. By recording receptive areas electrophysiologically and staining peripheral sensory neurites with horseradish peroxidase, we have shown that 'movement detector' neurites from one trigeminal ganglion do not normally cross the dorsal midline of the head to innervate areas of skin on the opposite side. However, if one trigeminal ganglion is removed before peripheral innervation starts, movement detector neurites from the intact side will now cross the midline to innervate contralateral skin. These observations suggest a specific competitive interaction between movement detector neurites during their innervation of head skin. The second class of receptor, 'rapid transient' detectors, have a different pattern of innervation, crossing the midline in both normal and operated animals.     

Studies of the action of symmetrical and mixed disulphide thiol-modifying reagents on the esterase activity of cytoplasmic aldehyde dehydrogenase

The effect of various thiol-modifying reagents on the esterase activity of sheep liver cytoplasmic aldehyde dehydrogenase is reported here. Both symmetrical reagents (disulfiram, 2,2'- and 4,4'-dithiodipyridines) and unsymmetrical reagents (methyl diethylthiocarbamyl disulphide, methyl 2- and 4-pyridyl disulphides) were investigated. The results suggest that all the modifiers react to varying extents with a pair of enzymic thiol groups ('A' and 'B'), and that the more specifically group 'A' is modified, the more the enzyme is inactivated. This supports the idea that group 'A' may be the essential nucleophile in the reaction catalysed by aldehyde dehydrogenase. Modification of group 'B' may or may not reduce the esterase activity depending on the nature of the label introduced. The results of the present experiments and of previous similar experiments concerning the dehydrogenase activity of the enzyme are consistent with the proposal that a common active site is responsible for both esterase and dehydrogenase activities.