A Rabbit Autoantibody Specific for the 46‐kDa Form of 2′,3′‐Cyclic Nucleotide 3′‐Phosphodiesterase

: An autoantibody occurring in the serum of an apparently normal rabbit that immunocytochemically stains myelin sheaths and oligodendrocytes in rat brain was shown to react specifically with the 46‐kDa isoform of 2′,3′‐cyclic nucleotide 3′‐phosphodiesterase (CNP) (EC 3.1.4.37) in a number of species. Identification of the shorter isoform of the enzyme (CNP1) as the antigen was achieved by comparing the immunostaining of Western blots by the autoantibody with that of a well‐characterized anti‐CNP antiserum. The 46‐kDa antigen reacting with the autoantibody exhibited the same Mr and pI as the small isoform of CNP on two‐dimensional gels and showed a similar enrichment in purified CNS myelin. The autoantibody has very high affinity for CNP1 and is capable of detecting the very low amounts of this enzyme in peripheral nerve, spleen, adrenal gland, pancreas, testis, and intestine. Testing the reactivity of the autoantibody with synthetic peptides by enzyme‐linked immunosorbent assay revealed that it reacted with the N‐acetylated decapeptide corresponding to the N‐terminus of CNP1, but did not react if the peptide was not acetylated or if the acetyl group was replaced with a palmityl group. The lack of reactivity with CNP2, which differs from CNP1 by a 20‐amino acid extension at the N‐terminus of the protein as a result of alternative splicing, may be due to the absence of the N‐acetyl moiety that is part of the epitope and/or blocking of antibody binding to the decapeptide by extension of the polypeptide chain. This highly specific antibody for the shorter isoform of CNP may be useful in studies to establish possible differences in the location or function of the two isoforms of this enzyme.