Electron microscopy of platinum-shadowed preparations of human tracheobronchial mucins showed very flexible filamentous structures that frequently occurred in an intricate random-coiled pattern of filament(s) surrounding a dense core-like domain. The filament(s) associated with cores accounted for 70-80% of the mass of the mucin preparation, the remainder being accounted for by free filaments. On aggregation, the molecules formed a large interwoven network quite different from the massive rope-like structures characteristic of sheep submaxillary mucin aggregates [Rose, Voter, Sage, Brown & Kaufman (1984) J. Biol. Chem. 259, 3167-3172]. Mild sonication resulted in extensive fragmentation of the tracheobronchial mucin molecules and yielded short filaments of various lengths, free cores and some cores associated with short filaments. Mucin glycopeptide fragments obtained by proteolytic digestion were flexible, core-free, filaments. The glycopeptides obtained by Pronase digestion were shorter than those obtained by tryptic digestion. The intricate structures of human tracheobronchial mucin differ markedly from the extended filaments reported for sheep submaxillary and human ovarian-cyst mucins but agree with the roughly spherical expanded model proposed for mucins by Creeth & Knight [(1967) Biochem. J. 105, 1135-1145] on the basis of hydrodynamic measurements.

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We previously reported that incubation of the appropriat'e sugar nucleotides with extracts obtained from Type XIV Diplococcus pneumoniae resulted in the formabion of glycolipids and serologically active polysaccharides (1, 2). During the course of these studies, we observed the formation of an unusual sugar nucleotide when the extracts were incubated with UDP-N-acetylglucosamine. As shown below, the new nucleotide is UDP-2-acetamido-4 amino-2,4,6-trideoxyhexose. The extra&s also transferred t)he cliamino sugar t,o endogenous lipid acceptors. While diamino sugars are found in a few antibiotics (3), the only report of the occurrence of a 2,4-diamino-2,4,6-trideoxyhexose is that by Sharon and Jeanloz (4) ; the unusual amino sugar was a component of a polysaccharide produced by a strain of Bacillus subtilis. The amino sugar, called bacillosamine, was obtained as the 4-N-acetyl derivative after acid hydrolysis of the polysaccharide. Recently, Zehavi and Sharon (5) have identified bacillosamine as 2,4-diamino-2,4,6-trideoxy-L-altrose. Whether bacillosamine is identical to, or a diastereoisomer of the amino sugar reported below has not yet been established. Two enzyme fract'ions were required for the synthesis of the UDP derivative of the diamino sugar. They catalyzed the following reactions: Fraction I UDP-,V-Acetylglucosamine-) UDP-2-acetamido-4-keto-2,6-dideoxyhexose (UDP-XI) Fraction II pyridoxal phosphate UDP-XI + glutamate (> a-ketoglutarate + UDP-2-acetamido-4-amino-2,4,6-trideoxyhexose

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