Major Australian-Antarctic plate reorganization at Hawaiian-Emperor bend time.

A marked bend in the Hawaiian-Emperor seamount chain supposedly resulted from a recent major reorganization of the plate-mantle system there 50 million years ago. Although alternative mantle-driven and plate-shifting hypotheses have been proposed, no contemporaneous circum-Pacific plate events have been identified. We report reconstructions for Australia and Antarctica that reveal a major plate reorganization between 50 and 53 million years ago. Revised Pacific Ocean sea-floor reconstructions suggest that subduction of the Pacific-Izanagi spreading ridge and subsequent Marianas/Tonga-Kermadec subduction initiation may have been the ultimate causes of these events. Thus, these plate reconstructions solve long-standing continental fit problems and improve constraints on the motion between East and West Antarctica and global plate circuit closure.

The effect of metal ions on the activity and thermostability of the extracellular proteinase from a thermophilic Bacillus, strain EA.1.

The proteinase from the extremely thermophilic Bacillus strain EA.1 exhibits maximum stability at a pH of approx. 6.5. In the presence of calcium ions the half-life at 95 degrees C of the enzyme at this pH was 17 min, and loss of activity followed first-order decay kinetics. The role of metal ions in the activity and stability of the enzyme was studied using the holoenzyme, the metal-depleted apoenzyme, and a zinc-enriched apoenzyme preparation. Zinc and calcium ions were the preferred bivalent cations for the active site and stabilization site(s) respectively. Stabilization by metal ions was not in itself a highly stringent process, but ions other than calcium which stabilized the enzyme generally had a concomitant inhibitory effect on activity. Inhibition and stabilization of the enzyme by cations were concentration-dependent effects and certain ions activated the apoenzyme but not the holoenzyme. Manganese(II) ions conferred some stability and also activated the enzyme, but in the latter case were not as effective as zinc ions. The results are discussed with reference to the ionic radii, co-ordination number and preferred ligand donors of the ions. Mercury(II) ions severely compromised enzyme activity and stability, and the effects of thiol-reactive agents suggest that thiol groups also have a role in enzyme integrity.

The histologic evaluation of the implant interface with heterograft and allograft materials--an eight-month autopsy report, Part II.

This paper presents post mortem histologic specimens of the implant/osseous tissue interface from a patient who had had simultaneous bilateral maxillary sinus augmentation and root-form implant placement eight months prior. Two implants were observed: One implant was totally submerged in bone and graft material (2E), and other implant (3E) was devoid of bone at the apex. Microscopic examination revealed that a bony interface existed around implant 2E but that implant 3E had minimal bony interface. It is suggested that the minimal bony interface was the result of minor implant movement induced by variations in barometric pressure on the apex of implant 3E, and that eight months would not have been enough healing time prior to loading for this patient.

Suspension mechanism of subperiosteal implants in baboons.

It has been noted that overloading of an implant can lead to the premature loss of implants (Jones et al., 1979; Travis and Jones, 1986). An understanding of the mechanism by which subperiosteal implants support masticatory loads could lead to appropriate design changes for best utilization of this mechanism and thus increase the predictability of subperiosteal implants (James, 1983). This study attempts to analyze the suspension mechanism in implants recovered from a baboon after 10 years' use.