Step 1 Heat the oven to 325 degrees. Butter a 10-inch tube pan. Combine the flour, baking soda and salt in a bowl and stir to blend. Set aside.
Step 2 Cream the butter with an electric mixer. Add 1 cup of the sugar and beat until light and fluffy. Beat in the eggs one at a time, scraping down the sides of the bowl after each addition.
Step 3Beat in one-fourth cup lemon juice and the zest of 3 lemons. Stir in the flour mixture and sour cream alternately, mixing with a rubber spatula until completely blended. Scrape into the prepared pan.
Step 4Bake 60 to 70 minutes, until a toothpick inserted in the thickest part comes out clean. Cool on a rack for 10 minutes, then turn the cake out onto a serving plate.
Step 5Combine the remaining one-third cup lemon juice and the remaining two-thirds cup sugar in a small saucepan and cook over medium heat, whisking constantly, until the sugar dissolves. Stir in the remaining lemon zest.
Step 6Prick the warm cake all over with a skewer. Spoon and brush the glaze over the top and all sides of the cake. Cool completely before slicing.