Bestatin was found to be a competitive inhibitor (with respect to the Leu-NA substrate) not only of the isolated microsomal and cytosolic leucine aminopeptidases (Leu-APm and Leu-APc) but also of the aminopeptidases (APs) present in membrane preparations (from mouse liver) and on the cell surface of L5178Y cells. Kinetic parameters indicate that cellular AP is identical to Leu-APm. To rule out the possibility that AP-B is involved in the inhibition reactions, comparable studies with amastatin were performed. Electrophoretical studies revealed the solubilized cell membrane bound AP to co-migrate with Leu-APm in polycrylamide gels. The activity of the separated membrane AP was inhibited by bestatin in situ. The cell membrane bound AP activity was found to be lowest in lymphocytes, higher in tumor cells and highest in bone marrow cells and macrophages. Using synchronized L5178Y cells, the AP activity changes during the cell division cycle; the lowest activity was determined during the G1-phase and 35% higher values were measured during the S/G2-phase The fluctuation of the cell surface associated AP activity parallels with changes in the number of bindings sites for bestatin.

A D-galactose-specific lectin, purified from the marine sponge Geodia cydonium, is present on the cell surface of mucoid cells, free choanocytes and choanocyte clusters, as revealed first, by the adhesion assay which is based on the formation of "rosettes" with erythrocytes, and second, by immunofluorescence studies. Using the same techniques no lectin could be identified on the surface of archaeocytes. Rosette formation was inhibited in the presence of 20 mM D-lactose as well as after preincubation of erythrocytes with purified lectin. Titration experiments in a hemagglutination assay showed that the highest level of extractable lectin (5% of the total protein) is found in mucoid cells, lower concentrations are determined in choanocyte clusters (0.07%), free choanocytes (0.05%) and archaeocytes (0.01%). Only the mucoid cells were found to synthesize lectin which is secreted and subsequently transferred to the cell surface of other cell types. As one consequence of the binding of the lectin to the cell surface of aggregation-deficient choanocytes or archaeocytes, the conversion of these cells to aggregation-susceptible ones is observed. These results support previous evidence that the lectin is involved in the reaggregation process of single cells in the homologous biological system.

Single cells of the marine sponge Geodia cydonium aggregate species-specifically in the presence of a soluble aggregation factor to form large cell clumps. A lectin isolated from the same sponge species does not cause agglutination of Geodia cells but agglutinates only cells from heterologous species (e.g. Tethya lyncurium, Hemimycale columella, Pellina semitubulosa, Cacospongia scalaris, Verongia aerophoba). The process of agglutination is independent of divalent cations (they do not affect the agglutination process at concentrations up to 50 mM), occurs at 2°C, causes a reduction in the viability of the cells and results in an inhibition of programmed syntheses. The observed differences between the properties of cell agglutination (effect of a lectin in a heterologous system) and cell aggregation (effect of an aggregation factor in the homologous system) is discussed. Cell aggregation is dependent upon the presence of an aggregation factor, the presence of cations and an incubation temperature 2̃0°C; cell aggregation results in a stimulation of programmed syntheses. Cell agglutination requires a heterologous macromolecule (e.g. lectin), it is independent of divalent cations and causes inhibition of programmed syntheses in the cells.

From the marine sponge Geodia cydonium a series of macromolecules have been isolated and characterized which are involved in the control of aggregation and separation of sponge cells; these include aggregation factor, aggregation receptor, anti-aggregation receptor, β-glucuronidase, β-glucuronosyltransferase, β-galactosyltransferase, β-galactosidase and a lectin. These components might be linked in the following sequence: (a) activation of the aggregation receptor by its enzymic glucuronylaion; (b) adhesive recognition of the cells, mediated by the aggregation factor and the glucuronylated aggregation receptor; (c) inactivation of the aggregation receptor by its deglucuronylation with the membrane-associated β-glucuronidase; (d) cell separation due either to the loss of the recognition site (glucuronic acid) of the aggregation receptor for the aggregation factor or to an inactivation of the aggregation factor by the anti-aggregation receptor. The activity of the anti-aggregation receptor is probably controlled by the Geodia lectin.

A macromolecule has been isolated from the cell membranes of the sponge Tethyalyncurium . This macromolecule was purified and found to bind to a d -galactose specific lectin from the sponge Geodia cydonium . The lectin receptor was characterized as a glycoprotein with a molecular weight in the region of 155 000 . Evidence is presented indicating that the binding of lectin with the lectin receptor is caused by hydrophobic interactions.

A lectin from the marine sponge GEODIA CYDONIUM was isolated and characterized. GEODIA lectin (GL) agglutinates human red blood cells irrespective of the ABO blood group and precipitates with a variety of D-galactose containing glycosubstances, i.e. certain snail galactans, bovine erythrocyte glycoprotein and PNEUMOCOCCUS type XIV polysaccharide. The only simple sugars inhibiting the GL-mediated hemagglutination were lactose and n-acetyl-D-galactosamine.

The aggregation receptor (AR) from the marine sponge GEODIA CYDONIUM was analyzed with respect to its monosaccharide composition. Three major sugars (D-galactose, D-glucose and D-glucuronic acid) accounted for about 85 % of the total carbohydrate. Negative results with different lectins directed against D-galactosyl, N-acetyl-D-galactosaminyl and N-acetyl-D-glucosaminyl groups, respectively, showed that these sugars are serologically unreactive in AR. Positive serological reactions were obtained with CONCANAVALIN A and LIMULUS POLYPHEMUS agglutinin. AR also reacted strongly with the basic polymer poly-L-lysine. Reaggregation experiments performed on the basis of these findings strongly suggest that AR-bound D-glucuronic acid is recognized by an intercellular aggregation factor (AF) during the secondary aggregation of GEODIA cells.

For the first time, the biological role of a lectin in the process of reaggregation of single cells from the same species (marine sponge: Geodia cydonium Jam.) is described. The galactose-specific lectin does not promote aggregation, but prevents the antiaggregation receptor from disaggregating cell clumps. Competition experiments showed that the lectin inactivates the antiaggregation receptor by binding to it, most likely via its terminal galactose residues. The lectin converts reversibly aggregation-deficient cells (carrying functional cell membrane-bound antiaggregation receptor molecules) to aggregation-susceptible cells.