Heat Lability of Transcobalamin II 1

Summary

⁵⁷Co B₁₂ labeled normal serum was heated at 56° for 30 min and chromatographed on DEAE-cellulose columns, using the following sodium phosphate buffers for elution: 0.0175 M, pH 6.3; 0.04 M, pH 5.9; 0.1 M, pH 5.8, and 0.4 M, pH 5.2. The amount of bound ⁵⁷Co B₁₂ eluted with 0.04 M buffer decreased and that eluted with 0.1 and 0.4 M buffer increased in heat-treated serum. Gel filtration indicated that 0.04 M eluate was possibly an aggregate of transcobalamin II (TCII). Plasma disappearance rate of this fraction was similar to that of TCII. Gel filtration of normal TCII, heated in solution at 56° for 30 min, indicated that ⁵⁷Co B₁₂ was in free form. Following heating of ⁵⁷Co B₁₂ labeled serum at 56° for 2 hr, 80% of the vitamin was unbound. It appeared that TCII is a heat labile substance.