Elucidation of the Exact Sites of Cleavage of Tropocollagen by Rheumatoid Synovial Collagenase: Correlation of Cleavage Sites with Fibril Structure

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Abstract
The site of attack of purified rheumatoid synovial collagenase on bovine and human tropocollagens has been determined by electron microscopic examination of dimeric and polymeric Segment-Long-Spacing (SLS) fragments produced after collagenase treatment at 27°C. No differences in the mode of attack of rheumatoid synovial collagenase on bovine or human tropocollagens were observed. The enzyme cleaved tropocollagen molecules in the apolar region between bands 41 and 42. (For line nomenclature, see Bruns and Gross1). Examination of junctions of both TC¾ and TC½ fragments of SLS showed this cleavage to be unique. The purified collagenase preparations also removed peptides from both N- and C-terminal ends of the tropocollagen molecule, although whether these cleavages were due to the collagenase itself or to trace amounts of contaminating peptidases is not certain. Pepsin treatment of TC¾ and TC¼ fragments cleaved a section about 60Å long from the N-terminal end of the TC¼ fragment–as far as band 44. No alterations in TC¾ fragments were observed. Comparison of the SLS line pattern with that of the native fibril showed the cleavage to be close to the fibril 4 line, i.e. close to the band-interband junction. The pepsin susceptible ‘weak’ region lies between the fibril 4 and 2 lines, right across the band-interband junction. (For fibril line nomenclature see Hodge and Schmitt2). It is suggested that rheumatoid synovial collagenase attacks the tropocollagen molecule at a ‘weak’ point, which, after initial cleavage, becomes susceptible to pepsin attack. In the fibril, this weak region is at the band-interband junction, probably adjacent to the C-terminal telopeptide of an adjacent 640Å staggered molecule, with which it may interact.