Table 1. C99S and C450S retain carboxylase activity that is propeptide-dependent.

	Specific activity, %
Carboxylase variant	Propeptide absent	Propeptide present
Wild type	100	100
C99S	2	14
C450S	19	40
Mock	0	0

Microsomes from viral- or mock-infected cells were solubilized in the absence or presence of propeptide (5 μM), and all of the preparations were then assayed for carboxylase activity in the presence of propeptide. Quadruplicate samples (duplicate aliquots of 10 or 20 μl) were assayed for 30 min. Background levels were determined by assaying a sample containing water instead of microsomes, which gave a value identical to that of the mock sample. The ratio of signal to background ranged between 150 (for wild-type enzyme) and 3 (for C99S). Microsomal aliquots were also subjected to quantitative Western blot analysis to determine the amount of carboxylase protein present and consequent specific activity. The variation in specific activities was <10%, and the entire experiment was performed twice. The values for cells solubilized in the absence of propeptide are higher than our previously determined values (23) because of the optimization of assay and isolation conditions.