CALCIUM-BINDING AND CONCOMITANT CHANGES IN THE STRUCTURE AND HEAT-STABILITY OF CALPROTECTIN (L1 PROTEIN)

Aim-To obtain further data on the structure and conformation of calpro tectin, a prominent leucocyte protein found in many species. Methods-T he binding of Ca2+ to calprotectin was studied by means of equilibrium dialysis using Ca-45 as tracer. The thermal stability and denaturatio n kinetics of calprotectin were studied by means of differential scann ing calorimetry. Concomitant alterations in optical activity resulting from different conditions were measured. A computer program calculate d the parameters to fit different models of protein structure. Ultravi olet spectroscopy gave absorbtion spectra. Sedimentation velocity stud ies and molecular weight determinations by the low speed (sedimentatio n) equilibrium technique were performed. Results-A maximum of six calc ium ions were bound per calprotectin molecule at 0.7 mM calcium chlori de. The apparent dissociation constants were calculated. Ca2+ ions inc reased the denaturation temperature by 26 degrees K. The enthalpy of d enaturation was also increased by Ca2+ Addition of Ca2+ to the buffers caused a gradual change in the near UV circular dichroism spectrum, w hile only minor changes were seen at wavelengths of 210-240 nm. A grad ual increase in the sedimentation coefficient was observed on addition of calcium chloride. The extinction coefficient at 279 nm was determi ned: E(279) = 2.53.10(4) M(-1) cm(-1). Conclusions-Calprotectin can bi nd six calcium ions. Upon binding, the protein shows distinct conforma tional changes and increased thermal stability. The former may be of i mportance for its function, while the biological significance of the l atter is unknown.