[Biochemical studies on collagen and connectin from human skeletal muscle: age-related changes in the properties of elasticity (author's transl)].

Collagen and connectin, an intracellular elastic protein, play important roles in structural maintenance and mechanical continuity of skeletal muscle. The author has investigated age-related changes in these proteins from human skeletal muscles ranged from 6 lunar month to 79 years of age. The total content of collagen in skeletal muscle at 6 lunar month was 1.7% of wet weight of the tissue. This value was found to decrease to 0.8% by 5 years of age, and no significant change in this value was observed after 5 years of age. Collagen in skeletal muscle tissue was found to consist of 80% of Type I collagen, 19% of Type III collagen and 1% of Type IV collagen. The proportion of these genetically distinct types of collagen was constant throughout the whole life-span. The amount of soluble collagen and formation of reducible cross-links of collagen were found to decrease with age. Although the content of connection in skeletal muscle from human embryo was low, 0.9-1.1% of wet weight of the tissue, it gradually increased to 2.9% at 79 years of age. Following NaB3H4 reduction, connectin was found to contain the reducible cross-links derived from lysine- and hydroxylysine-aldehydes; two of them were identified as lysinonorleucine and histidino-hydroxymerodesmosine, and additional three reducible compounds were eluted between elution positions of dihydroxylysinonorleucine and lysinonorleucine from ion exchange chromatography. These findings strongly indicate that connectin and connective tissue proteins, collagen and elastin, share common features of cross-linking. Furthermore, these reducible cross-links were found to decrease with age. Thus, maturations of both connectin and collagen are apparently due to qualitative changes in their reducible cross-links.