Multicompartmental model describing secretion and intracellular degradation of apoB. Compartments 1–8, 10, and 12 represent intracellular apoB, whereas compartment 9 represents apoB secreted into the media. Compartment 1 represents the radioactive amino acid tracer pool. Compartments 2–7 represent incorporation of tracer into progressively longer fractional lengths of nascent apoB during translation; compartment 7 represents the first appearance of a pool of full-length protein. ApoB can be lost from compartment 7 via a rapid presecretory degradation pathway or can pass on to compartment 8 or 10. Loss of apoB from compartment 10 occurs through a slow presecretory degradation pathway. Compartment 8 represents the delay in apoB appearance in the media; compartment 12 represents the experimentally observed extended delay in apoB100 appearance in the media relative to apoB48. Loss of apoB from compartment 8 occurs via a degradation pathway that is inhibited by heparin addition. Compartment 9 represents apoB in the media. Experimentally determined apoB radioactivity is indicated by the hatched compartments 7–10.