TRANSFERRIN AND THE TRANSFERRIN CYCLE IN BELGRADE RAT RETICULOCYTES

Belgrade rats have an autosomal recessive anemia with hypochromia and microcytosis. Iron uptake into reticulocytes is approximately 20% of n ormal, but transferrin uptake is unimpaired. We have systematically co mpared the transferrin cycle in Belgrade versus normal reticulocytes t o locate the defect more precisely. Belgrade transferrin was functiona lly normal as purified transferrin or whole plasma. Transferrin affini ty of Belgrade receptors was indistinguishable from normal, but Belgra de reticulocytes had twice as many receptors. Belgrade transferrin end ocytosis was 1.5 times faster than normal, whereas exocytosis is about twice as fast. Initially Belgrade reticulocytes internalize iron at a n unimpaired rate, but they lag behind normal by 5 min. During reincub ation, they release 25-33% of iron taken up during a 30-min preincubat ion, whereas normal cells do not lose a detectable fraction. Unexpecte dly, transferrin cycle time was unchanged. Hence another kinetic step of the cycle is slower, compensating for increases in Belgrade endocyt osis and exocytosis. After one cycle, Belgrade reticulocytes retain on ly half of the iron that entered, but over 90% of iron entering normal cells remains within. Iron unloading is ineffective inside the Belgra de vesicle; 85% of iron that entered on transferrin returned to the me dium after exocytosis, whereas only 45% of iron entering normal reticu locytes exits. Ineffective utilization of iron in or near Belgrade end osomes accounts for the Belgrade defect.