Collagen constitutes about 12% in females and 17% in males of the total protein in mice

Collagen has been postulated to be the most abundant protein in our body, making up one-third of the total protein content in mammals. However, a direct assessment of the total collagen levels of an entire mammal to confirm this estimate is missing. Here we measured hydroxyproline levels as a proxy for collagen content together with total protein levels of entire mice or of individual tissues. Collagen content normalized to the total protein is approximately 0.1% in the brain and liver, 1% in the heart and kidney, 4% in the muscle and lung, 6% in the colon, 20–40% in the skin, 25–35% in bones, and 40–50% in tendons of wild-type (CD1 and CB57BL/6) mice, consistent with previous reports. To our surprise, we find that collagen is approximately 12% in females and 17% in males of the total protein content of entire wild-type (CD1 and CB57BL/6) mice. Although collagen type I is the most abundant collagen, the most abundant proteins are albumin, hemoglobulin, histones, actin, serpina, and then collagen type I. Analyzing amino acid compositions of mice revealed glycine as the most abundant amino acid. Thus, we provide reference points for collagen, matrisome, protein, and amino acid composition of healthy wild-type mice.