What does pre-emphasis $\alpha=1$ will do to a signal?
The pre-emphasis filter is a simple high-pass filter with a monotonically increasing magnitude of its frequency response $H(\omega)$. Its value at DC is $H(0)=1-\alpha$, and its value at Nyquist (i.e., the maximum frequency) is $H(\pi)=1+\alpha$. Consequently, for $\alpha=1$ you get a zero at DC, which means that the filter will totally suppress a constant input signal (which is obvious from looking at the difference equation). 

Note that a pre-emphasis with $\alpha=1$ cannot be compensated for because the DC component is lost and can't be recovered. In signal processing terms: there is no stable inverse filter.