Isolation of type I collagen homotrimer from human placenta with LC–MS monitoring of the α1(I)/α2(I) chain ratio

The general molecular form of type I collagen is heterotrimer consisting of two α1(I) chains and one α2(I) chain. However, α111(I) homotrimer is rarely observed in vivo, especially in pathological tissues such as cancer. Here we utilized a previously developed LC–MS method that can accurately and sensitively quantitate α1(I) and α2(I) chains to distinguish type I collagen homotrimer from human placenta. By monitoring with the LC–MS method, the α1(I)/α2(I) chain ratio was found to be high in the supernatant of salt precipitation with >2.8 M NaCl at neutral pH. Type I collagen homotrimer was successfully isolated using optimized sequential salt fractionation and confirmed to show previously reported features of the homotrimer, including high thermal stability and overmodification. These data clearly indicate that placental tissue contains α111(I) homotrimer. Our LC–MS method can sensitively detect the rare form of type I collagen and can help understand its physiological and pathological significance.