Hen oviduct signal peptidase requires only two proteins for proteolysis of fully synthesized secretory precursor proteins in vitro: one with a molecular mass of 19 kilodaltons (kDa) and one which is a glycoprotein whose mass varies from 22 to 24 kDa depending on the extent of glycosylation. Purified signal peptidase has been analyzed both as part of an active catalytic unit and after electroelution of the individual proteins out of a preparative polyacrylamide gel. The multiple forms of the glycoprotein component of signal peptidase bind to concanavalin A and are shown to be derived from the same polypeptide backbone. Removal of their oligosaccharides by digestion with N-glycanase converts these proteins to a single 19.5-kDa polypeptide. The glycoproteins all exhibit very similar profiles following individual digestion with trypsin and separation of the resulting peptides by reverse-phase high-performance liquid chromatography. In addition, sequence analysis of selected peptides from corresponding regions in chromatograms representing each form of the glycoprotein reveals the same amino acid sequences. The 19-kDa signal peptidase protein does not bind concanavalin A, has a distinct tryptic peptide map from that of the glycoprotein, and appears to share no amino acid sequences in common with the glycoprotein. Its copurification on a concanavalin A-Sepharose column indicates that it must interact directly with the glycoprotein subunit.

The incidence of hypophosphataemia was somewhat elevated in HOPS patients who took TDF-containing HAART compared with those who took TDF-sparing HAART during the first 1 to 2 years of observation, but the difference was not statistically significant. Longer follow-up of a larger population is needed to determine if this trend towards an association achieves statistical significance and to evaluate the clinical consequences of hypophosphataemia.

Exposure of horse platelets to thrombin has been reported to cause nearly complete inactivation of cyclooxygenase within 30 sec. This contrasts with the observation that human platelets, depleted of their granule constituents by stimulation with thrombin, still aggregate in response to arachidonic acid, a reaction presumably mediated by thromboxane A2 (TxA2) formation. Because of this conflicting evidence, TxA2 formation was measured by radioimmunoassay in washed human platelets depleted of their alpha- and dense-storage granule constituents by prior stimulation with thrombin. These platelets aggregated in response to adenosine diphosphate (ADP), collagen, arachidonic acid, and thrombin, and formed TxA2. However, collagen- and thrombin-induced TxA2 formation by these platelets was reduced in comparison to control platelets that had not been depleted of their storage granule constituents by prior thrombin stimulation. In contrast, arachidonic acid-induced TxA2 formation was not significantly different in thrombin-depleted and control platelets. These results demonstrate that thrombin can induce degranulation of platelets without concomitant inactivation of cyclooxygenase.

We previously demonstrated that platelets can be labeled with 111Inoxine with high labeling efficiency and that 111In is not liberated from labeled platelets during the platelet release reaction or prolonged in vitro storage. In view of these findings, we examined the potential usefulness of loss of 111In from labeled platelets as an indicator or platelet damage by comparing the loss of 111In with that of 51Cr and LDH (in some experiments also with platelet factor 3 availability) under different conditions of platelet injury. When washed human platelets labeled with either 51Cr-chromate or 111In-oxine were exposed to increasing concentrations of detergents (Triton X-100, lysolecithin), threshold, rate, and extent of loss of 111In, 51Cr and, LDH were similar. In contrast, when labeled platelets were depleted of metabolic energy by incubation in glucose-free Tyrode albumin solution or glucose-depleted plasma in the presence of antimycin A and 2-deoxy-D- glucose, loss of 51Cr (and PF3a) occurred earlier and progressed at a faster rate than that of 111In or LDH. Similar results were obtained when platelets were exposed to increasing concentrations of PlA1 antibody, causing complement-mediated immune injury. The findings indicate that with certain agents that cause rapid platelet disruption (lysis), different platelet constituents are lost at similar rates. However, under conditions of more subtle or slowly progressive platelet injury, small molecules such as adenine nucleotides (51Cr) may escape earlier and at faster rates than larger molecules such as LDH or 111In- binding platelet protein. Thus, neither 111In loss nor LDH loss appear to be suitable indicators for sublytic or prelytic platelet injury.

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