Here is the Z420 motherboard P1 connector pinout:
The +12V, -12V, and Ground pins are obvious, the +11V_AUX_PSU is an auxiliary voltage, PSON# is the power switch, pin 16 is a fan PWN signal, and pin9 lets the system know what wattage power supply is installed.
Note: The Z420 power supply is desiged to provide several +12V rails to the motherboard. The motherboard +5V and +3.3V (and others) are created from these +12V rails.
** The +5V pins do not supply power. They are connected to the motherboard +5V regulator, meaning that the motherboard routes +5V power to the power supply. **
It looks like the adapter that Anu0512 posted only connects the ATX 12V power to the Z420. Because the Z420 creates other voltages from the +12V rails, an ATX power supply might not have enough +12V power to support a fully configured Z420 system. FYI.