Further biochemical and biophysical characterization of two low-molecular-weight, strongly acidic proteins that are present at extremely high levels in the organ of Corti, tentatively named OCP-I and OCP-II, is presented. The two proteins are also present, although at much lower levels, in the vestibular end-organs and a variety of other inner ear tissues; they have not been observed in other systems. OCP-I and II are highly soluble and do not contain appreciable amounts of carbohydrate. The two proteins, originally described in the guinea pig, are compared electrophoretically with the corresponding proteins in several other mammalian species. Preliminary data on the amino acid composition of the two proteins are presented. Moreover, the amino-terminal sequence of a 22-residue segment of OCP-II is shown and compared to the sequences of known proteins.
Two highly acidic, low molecular weight proteins, present in very high concentration in the organ of Corti (OC) and termed OCP-I and OCP-II [I. Thalmann et al., Arch. Otorhinolaryngol. 226, 123–128 (1980)] have been previously described. The proteins do not contain significant amounts of carbohydrate and are soluble in both high- and low-ionic strength buffer. The distribution of these unidentified proteins in different cell types of the OC has now been determined and has been compared with the distribution of actin, tubulin, and calmodulin. Freeze-dried and freshly dissected substructures of the OC of chinchilla and guinea pig were used, respectively. Actin was found to be particularly abundant in the inner and outer pillar cells. In the Hensen and Deiters cells, OCP-I and OCP-II predominate and calmodulin is present only in very small quantities. In contrast, the outer hair cells exhibit a strong accumulation of calmodulin, with relatively low levels of OCP-I and OCP-II, actin and tubulin. In the neuroepithelium of the sacculus OCP-I and OCP-II are present at about 40 times lower concentrations than in the OC.
Two highly acidic, low molecular weight proteins, present in very high concentration in the organ of Corti (OC) and termed OCP-I and OCP-II [I. Thalmann et al., Arch. Otorhinolaryngol. 226, 123–128 (1980)] have been previously described. The behavior of these proteins during the post-natal development in the rat cochlea has now been determined. On the 6th day post partum (pp), only trace amounts of the substances are present. The concentration of OCP-I and OCP-II increases most rapidly between days 12 and 14 pp, and the proteins approach adult levels by day 18 pp. The sharp increase of OCP-I and OCP-II occurs at approximately the same time as the sudden increase of the endolymphatic potential on day 13 pp [S. K. Bosher and R. L. Warren, J. Physiol. 212, 739–761 (1971)]. It is of interest that a major basilar papilla-specific protein in the chick increases in step with the onset of auditory function during embryonic development [J. C. Oberholtzer et al., Hear. Res. 23, 161–168 (1986)]. This protein is similar but not identical in its electrophoretic pattern to OCP-II. The behavior of OCP-I and OCP-II will be compared with the pattern of other important proteins such as actin, tubulin, and calmodulin.
Previously it has been reported that levels of most amino acids in small perilymph samples (< 100 nL) taken from ST through the round window (RW) of guinea pigs are distinctly different from those in CSF [A. N. Salt and R. Thalmann, Adv. Otorhinolaryngol. (in press)]. The composition of samples of larger volume more closely resembles CSF. Now an analysis of amino acid content has been performed as the sample volume withdrawn through the RW was systematically varied from 100 nL to 5 μL. Glycine, serine, glutamine, and alanine were measured by HPLC (Physiological Pico-Tag, Waters). Up to 200 nL, the glycine content of ST perilymph was similar to that of scala vestibuli. With larger volumes, the glycine content rapidly declined toward that of CSF. These data suggest that perilymph samples taken through the RW are contaminated with CSF if more than 200 nL is withdrawn. If ST perilymph is sampled through a small fenestra in the third turn, larger volumes (up to 2μL) can be withdrawn without significant CSF contamination, evidently because of the distance from the cochlear aqueduct. The data also suggest that glycine content can be used as an indicator of CSF contamination of ST perilymph samples.
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