Sr-containing carbonate apatite (SrCAp) specimens of varied Sr contents, ranging from 0 to 13.3 mol%, were prepared through a phosphate treatment of set gypsum-and-carbonate mixture at 100°C for 7 days. Effects of Sr content in SrCAp on microstructure, osteoblast-like cell (MC3T3-E1) attachment and proliferation, and alkaline phosphatase (ALP) activity were evaluated. Sr 2+ ion substituted Ca 2+ ion in the apatite lattice. Carbonate content was about 9-13.6 wt%, increasing in content level as Sr content increased. Sr addition benefited cell attachment but had no significant influence on cell proliferation, although the latter was inhibited at the highest Sr content. ALP activity reached a peak in specimen containing 3.4 mol% of Sr. The present study revealed that SrCAp is a promising candidate for use as a bone substitute material with good resorbabilty and osteoconductivity.
Threonine synthase catalyses the conversion of O-phospho-l-homoserine and a water molecule to l-threonine and has the most complex catalytic mechanism among the pyridoxal 5′-phosphate-dependent enzymes. In order to study the less-characterized earlier stage of the catalytic reaction, we studied the reaction of threonine synthase with 2-amino-5-phosphonopentanoate, which stops the catalytic reaction at the enamine intermediate. The global kinetic analysis of the triphasic spectral changes showed that, in addition to the theoretically expected pathway, the carbanion is rapidly reprotonated at Cα to form an aldimine distinct from the external aldimine directly formed from the Michaelis complex. The Kd for the binding of inhibitor to the enzyme decreased with increasing pH, showing that the 2-amino-group-unprotonated form of the ligand binds to the enzyme. On the other hand, the rate constants for the proton migration steps within the active site are independent of the solvent pH, indicating that protons are shared by the active dissociative groups and are not exchanged with the solvent during the course of catalysis. This gives an insight into the role of the phosphate group of the substrate, which may increase the basicity of the ε-amino group of the catalytic lysine residue in the active site.
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