The purine ring system is shown below.What is the source of the N-1 atom during de novo biosynthesis of purines? 
Definitions:
Variable Expense Per Unit
The cost that varies with each unit of product produced or sold.
Total Contribution Margin
The total amount of revenue remaining after all variable costs have been subtracted, indicating how much revenue is available to cover fixed costs and generate profit.
Sales Volume
The quantity of goods or services sold within a specific period.
Break-even Point
is the level of sales at which total revenue equals total costs, resulting in neither profit nor loss for the business.
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