Degradation of enamel matrix proteins in porcine secretory enamel

To elucidate the progressive disappearance of 25 kDa amelogenin occurring i n a narrow space near the surface of enamel, the alkaline soluble fraction which contained 80% of the total proteins was extracted from a newly formed porcine enamel. When this fraction was incubated with the addition of Ca i ons in an in vitro system, the degradation of the coexisting amelogenin and enamelin occurred without activation during the incubation period. Althoug h the fraction contained mainly two kinds of metalloproteinases, 56 kDa and 61 kDa gelatinolytic, and 41 kDa and 46 kDa caseinolytic activities, it wa s demonstrated on amelogenin enzymography that the caseinolytic one was con cerned with the conversion of the 25 kDa amelogenin into the 20 kDa ameloge nin, The protein distribution of the newly formed enamel indicated that the metalloproteinases degraded the coexisting enamelin and amelogenin imperfe ctly. Nevertheless, during the next developing stage they demonstrated thei r full activities. It is suspected that these activities are regulated by C a ions, which may be increased by a cascade system.