Measurment of rhesus monkey (Macaca mulatta) apolipoprotein B in serum by radioimmunoassay: comparison of immunoreactivities of rhesus and human low density lipoproteins

A sensitive and specific double antibody radio- immunoassay for the major apolipoprotein (apoB) of rhesus (Macaca muIatta) serum very low density lipoprotein (VLDL) and low density lipoprotein (LDL) is described. The anti- serum was raised to LDL (d 1.030-1.040 g/ml) and the LDL, (d 1.020-1.050 g/ml) was labeled with '=I by the chloramine-T or iodine monochloride method. The assay, which was sensitive to 0.02-0.5 pg of LDL2, had an inter- assay coefficient of variation of 4.5%. This assay was suc- cessfully used to measure apoB in the whole serum and low density lipoproteins of control monkeys maintained on a standard Purina monkey chow (PMC) diet and of three groups of monkeys fed atherogenic diets: an "average American diet," a 25% peanut oil and 2% cholesterol-sup- plemented PMC diet, and a 25% coconut oil and 2% choles- terol-supplemented PMC diet. The control monkeys (n = 13) had a serum cholesterol of 146 f 28 mg/dl and an apoB of 50 f 18 mg/dl. In the monkeys maintained on the atherogenic diets the serum apoB was elevated: 103 +- 28 mg/dl (American), 102 f 35 mg/dl (peanut oil), and 312 f 88 mg/dl (coconut oil). The values for serum total choles- terol were 333 f 65 mg/dl (American), 606 f 212 mg/dl (peanut oil), and 864 f 233 mg/dl (coconut oil) and were elevated relative to controls (P < 0.001). For each of the diets, total serum cholesterol correlated with serum apoB (E' < 0.001). The slopes of the regression lines of serum apoB vs. cholesterol for the monkeys on the PMC, Ameri- can, and coconut oil diets were similar (m = 0.531, 0.401, and 0.359, respectively), but differed from that of monkeys on the peanut oil diet (m = 0.12 1). The immunoreactivities of rhesus and human LDL were compared using specific antisera raised against these antigens. In homologous assay systems, monkey and human LDL exhibited unique im- munological determinants. The same results were obtained with the delipidated preparations of the two LDLs using antisera raised against either monkey or human apoB. Crossover studies using a heterologous tracer with each anti- serum resulted in the selection of a specific population of antibodies directed against antigenic sites shared by these two LDL species.