1. The surface behaviour of six different gangliosides and eight chemically related glycosphingolipids was investigated in monolayers at the air-water interface. 2. Mono-, di-, tri and tetra-hexosylceramides had force-area isotherms showing similar limiting molecular areas on 145 mM-NaCl, pH 5.6. The increasing number of negatively charged sialosyl residues in mono-, di- and tri-sialogangliosides induced a progressive increase in the liquid-expanded character of the films and in the limiting area occupied per molecule, owing to electrostatic repulsions. When the ganglioside monolayers were spread on subphases at pH 1.2, the limiting area per molecule was similar to that found for neutral glycosphingolipids. 3. The monolayer collapse pressure at pH 5.6 increased with the number of uncharged carbohydrate units up to when the polar head group contained 3-4 residues. For gangliosides the collapse pressures were lower and decreased from mono- to tri-sialogangliosides. Ganglioside monolayers on subphases at pH 1.2 showed increases in their collapse pressure. 4. The glycosphingolipid monolayers studied had various surface in their collapse pressure. 4. The glycosphingolipid monolayers studied had various surface potentials according to the complexity of the polar head group of the lipid. Attempts to calculate the dipolar contributions to the surface potential from each carbohydrate residue suggest that the second and third sialosyl residues in di- and tri-sialogangliosides contributed with a vertical dipole moment opposite to that of the first sialosyl residue.

A 100,OOO g supernatant fraction from rat brain that was passed through a column Receipient of a scholarship from the Instituto Nacional de Farmacologia y Bromatologia dependiente de la Secretaria de Salud Pliblica de la Nacion. Argentina. 97

The product of the incorporation of [14C]tyrosine as single unit into a protein of the soluble fraction of rat brain homogenate was purified by following a procedure used to purify tubulin. Sodium dodecylsulphate -polyacrylamide gel electrophoresis of the purified material showed a single protein band containing all the radioactivity. Purification data indicate that this protein accounts for 10.2 of the total protein of the supernatant fraction. This is in good agreement with the amount found for tubulin by the [3H]colchicine-binding method (10.5 % of the total protein). The incorporated ['"CItyrosine was found in the a-subunit of tubulin. Protein labelled with [3H]colchicine and [14C]tyrosine was precipitated with vinblastine sulphateand the radioactivity of 3H and that of 14C were quantitatively recovered in the precipitate (98 %).Sodium dodecylsulphate -polyacrylamide gel electrophoresis of the vinblastine precipitate showed that the I4C radioactivity moved with the tubulin band.Results obtained in experiments with phenylalanine and 3,4-dihydroxyphenylalanine were identical to those obtained for tyrosine.Binding of colchicine did not interfere with the incorporation of tyrosine.

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