Biochemical relationships between endolymph and otolith matrix in the trout (Oncorhynchus mykiss) and turbot (Psetta maxima)

This paper compares the organic compositions of the otolith and endolymph of trout and turbot. Irrespective of the method of demineralization (0.5 M EDTA or acetic acid), trout otoliths were found to be largely composed of proteins (48%), collagens (23%), and proteoglycans (29%). Collagen was only detectable in the EDTA-insoluble (0.30 mug/mg) and in the acetic acid-soluble fractions (0.53 mug/mg). The same compounds were found in the endolymph but in different proportions (proteins 85%, collagens 12%, and proteoglycans 3%). It was shown that the distribution of these compounds was not uniform within the endolymph. Proteins, collagens, and amino acids were 4, 10, and 3 times, respectively, more concentrated in the proximal (facing the macula) than the distal side whereas proteoglycans were 10 times more concentrated at the distal side. SDS PAGE analyses of proximal and distal samples of endolymph showed similar patterns suggesting that the spatial gradient of protein is quantitative and not qualitative. SDS PAGE comparison of endolymph and otolith samples showed only two proteins with similar molecular weights. We propose that collagen and protein gradients are involved in the organic matrix formation and otolith calcification process. Endolymphs from both trout and turbot display inhibitions of in vitro calcification although these inhibitions were 50 and 80 times, respectively, less than that of the otoliths. The inhibitory factor probably plays a significant role in the regulation of otolith calcification.