Methods Male Sprague-Dawley rats weighing 150 to 200 g were used (Charles River Breeding Laboratories, North Wilmington, Mas-sachusetts). Materials were purchased as follows: chromatographically purified neuraminidase (Clostridium perfringens) and soybean trypsin inhibitor (SBTI) from Worthington Bio-chemical Corporation, Freehold, New Jersey; puromycin aminonucleoside (PAN)(6-dimethylamino-9 [3'amino-3'deoxyribosyl purine]) from Sigma Chemical Company, St. Louis, Missou-ri; horse spleen ferritin, two times crystallized (cadmium free) from Calbiochem, San Diego, California; and bovine serum albumin (BSA) from Reheis Chemical Company, Phoenix, Arizona. Colloidal iron (CI) was prepared by the method of Rinehart and Abui-Haj [9].

Preparation of neuraminidase perfusates. Neuraminidase perfusates, at concentrations of 0.5 U/mi (40 pgIml) and 1.0 U/mi (80 g/m1) were prepared in a vehicle of 0.1 M sodium chloride acetate buffer, pH 5.4, containing 0.5 pg/mi and 1.0 g/ml of SBTI, respectively. SBTI was included to abolish any contaminating tryptic activity. The perfusates were prepared immediately prior to use and within a few days after the enzyme was received since storage of the enzyme for even a short period of time led to a marked and variable reduction in its enzymatic activity.